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The Isle of Portland is a notable and impressive place for geology, and with varied and contrasting scenary all within a small area. The top of the island ranges from fields, quarries, Admiralty buildings, small housing estates and interesting old villages and churches. The promontory is surrounded by excellent cliff exposures of Portland Stone, Purbeck strata and the top of the Kimmeridge Clay (as an example, the photograph above is of the cliffs south of Mutton Cove ). In the cliffs are raised beach deposits. From the cliffs to the centre of the "island" are numerous quarries, some working, many abandoned and these too provide excellent exposures. There are many interesting features such as fissures containing bones, fossil tree remains, celestite and gypsum, caves, ancient soils and numerous molluscan fossils. Dinosaur footprints of excellent quality are special features of Portland. While looking at the geology, historic features of Portland quarrying will be encountered and, in passing, it is a place of interest for shipwrecks, old castles, prisons, supposed sea-monsters and even a mythical weird bird!.

Before the localities are described, some technical details of the geology are provided, mainly for reference. Some readers may wish to skip over these and move on to the site descriptions which follow. Certain parts of Portland are described in associated webpages, listed at the top of this page. Progressively more localities on Portland will be described and more detail and illustrations will be added to the existing webpages.

Geological (and some other) publications on Portland are listed in the Portland Bibliography . Particularly see the Geologists' Association Guide by the late Professor Michael House . A key recent work on geomorphology is that of Brunsden et. al. (1996). A Portland Fieldwork Guide for schools and colleges, with supplementary case studies are available from Weymouth and Portland Borough Council. See Portland Bibliography for more details.

PORTLAND PART 1 - INTRODUCTION
Safety on Field Trips on Portland

Some general information regarding safety on Dorset geological field trips is provided and you are requested to read this if going to localities described here.

Some specific hazards with regard to Portland are mentioned here. Like almost everywhere else on the Dorset coast care must be taken with regard to hazard of falling rocks. On the Isle of Portland safety helmets should be worn beneath cliffs and in quarries, and places where it is clear that rock has recently fallen should be strictly avoided. Quarry regulations must be followed if these are visited and care taken to keep clear of heavy plant and machinery. It is important not to hammer the chert in the Portland and Purbeck strata because dangerous splinters can easily penetrate the body and may cause blindness. Take care not to fall down fissures in the Portland Stone and do not enter fissures or caves without the normal caving safety precautions. The low ledges around Portland Bill can be dangerous in rough sea conditions and should be avoided then. There is real risk of being swept off a ledge in stormy weather. The upper slopes of the cliffs, particularly on the west side of Portland are at a moderate angle but are above vertical Portland Stone. These upper slopes are hazardous and should be treated with great care. In general avoid cliff-climbing and keep away from the sea and its dangerous undertow on the Chesil Beach.

Celestite deposits of the western cliffs of Portland are described in these webpages. These are poorly exposed in recesses in steeply sloping cliffs above vertical cliffs. The celestite should not be approached without knowledge of the relatively safer places of access, and even then great care should be taken not to slip to the vertical cliffs beneath.

PORTLAND - PART 1 - INTRODUCTION
1.1 The Portland SSSI

The Isle of Portland is an important Site of Special Scientific Interest, and the protection which this affords is of great value. The Isle of Portland SSSI was notified in 1987 to the Weymouth and Portland Borough Council, Dorset County Council. The interesting geology is only a part of its special qualifications for inclusion. Information on the SSSI was kindly provided by Victoria Copley of the Dorset Team of English Nature at: English Nature, Slepe Farm, Arne, Wareham, Dorset, BH20 5BN. I am very grateful for her help. Some notes on the SSSI from the citation document are given below, but more information is available from English Nature.

The Island has a rich limestone flora. Some unquarried areas hold long-standing, unimproved gassland supporting a diverse range of plants but there is also considerable botanical interest in many of the old quarry workings which have become colonised by limestone plants. Shorter swards which are maintained by grazing and exposure to the wind are dominated by Sheep's Fescue Festuca ovina or Red Fescue Festuca rubra.. ...(continues on flora, mollusca, butterflies and birds - in all one A4 page of small print).

(Note: although good "fossil forests" occur on Portland the most well-know "Fossil Forest" is east of Lulworth Cove. For "stage", which is a subdivision of strata, read "Age" which is a subdivision of time)

PORTLAND, - GENERAL GEOLOGY
The Geological Succession

This classic diagram, based on Arkell (1933) shows the general uppermost Jurassic/basal Cretaceous succession in the Isle of Portland, compared with that on the Isle of Purbeck. The Isle of Portland is mostly composed of Upper Jurassic marine strata with a small thickness of basal Cretaceous Purbeck Formation. The Upper Jurassic, Kimmeridge Clay occurs beneath Portland Harbour and Castletown and is under the foot of the high northern cliffs on both sides. The Portland Sand, which is, in fact, largely marls with some sandy horizons, lies above. The Portland Stone consists of the Portland Cherty Series overlain by the Portland Freestone, mostly oolitic limestone. The Purbeck sequence follows. In the southermost part of the Isle of Portland there is a raised beach (or beaches) of Pleistocene age.

This classic sequence of clays and limestones has been described by Arkell (1933; 1947) and many other authors. Sedimentology has been discussed by Townson (1975), West (1975), Bosence (1987) and others. Note that there are correlation problems and arguments referred to in the section on Zones of the Portland Group, below.

Two alternative schemes for the terminology of the Portland and Purbeck successions are given here. Townson (1975) introduced a largely new terminology. However, it has not been widely used and the traditional scheme of Arkell, shown on the left is still in more common use (diagram after Bosence, 1987, from Townson, 1975). See Wimbledon (1987) for some discussion of this, and note also that some correlation problems which affect this are discussed in the section on zones, below.

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2.2 The Ammonite Zones of the Portland Group

Wimbledon and Cope (1978) revised the ammonite zones of the Portland Group. Their ammonite zones are listed below with the oldest at the bottom.

(It is interesting to note that because of the size and weight of these fossils, helicopters from HMS Osprey on Portland were used for lifting specimens up from the cliffs of Portland. These were zonal ammonite studies on a big scale! I know the nature of the ammonite transport problem, myself, because in my youth I cycled from Swanage to Bournemouth with a large Titanites on my back).

Titanites anguiformis Zone.
Index species: Titanites anguiformis Wimbledon and Cope 1978
The Portland Freestone mostly belongs to this zone (although the Portland Freestone of Purbeck may start earlier, in the kerberus zone). T. anguiformis, the fine-ribbed, evolute Titanites is probably the common giant ammonite of the Portland Stone. Caution with giant ammonites is needed, however, because Wimbledon and Cope (1978) point out that the giant ammonite Titanites giganteus (and Titanites titan) occurs in the Basal Shell Bed of the kerberus Zone. The Titanites anguiformis fauna has wrongly been referred to in the past as the giganteus fauna (see Wimbledon and Cope, 1978 for more details). The reference section for the base of the zone is on the south side of Freshwater Bay, Isle of Portland (SY 691700) at a point 3m below the top of the Cherty Beds (Wimbledon and Cope, 1978).

Galbanites (Kerberites) kerberus Zone.
Index species: Galbanites (Kerberites) kerberus (Buckman)
This is common in the Cherty Beds of Portland and Upper Cherty Beds of the Isle of Purbeck (note the complication - not Lower Cherty Beds of Purbeck). It occurs from the Basal Shell Bed of Portland and from the Prickle Bed or Puffin Ledge (J-J1) of the Isle of Purbeck according to Wimbledon and Cope (1978). The reference section is the northern side of Freshwater Bay, Isle of Portland (SY 692703), where the bottom of the basal unit of the Basal Shell Bed is taken as the base of the zone.

Galbanites okusensis Zone.
Index species: Galbanites okusensis (Salfeld)
This is present in the Black Dolomites ("Black Sandstones") of Purbeck and the upper part of the West Weare Dolomites ("West Weare Sandstones") of Portland and the Portland Clay where present above these. The dolomitisation renders the fossils poorly preserved and difficult to extract. Thus Wimbledon and Cope have defined the base of the zone in the Swindon region and not on Portland.

Note the complication that Wimbledon and Cope (1978, p. 187) stated that G. okusensis had not been found in the upper part of its zone. In Dorset the upper part of the zone was apparently established on the occurrence of Titanites (Polymegalites) polypreon because this ammonite is associated with G. okusensis in the Swindon reference section. Perhaps G okusensis has since been found in the upper part. If not, the placing of the Lower Cherty Beds of the Dorset mainland (Isle of Purbeck) in the okusensis zone, that is older than the lowest Cherty Series on Portland, is not based on direct evidence of the occurence of the zonal index but on circumstantial evidence. Townson (1975), Wimbledon and Cope (1978) and Wimbledon (1987) have all correlated the Prickle Bed (J1) of Purbeck with the Basal Shell Bed of Portland which would support the theory. It is worth drawing attention to the fact, though, that there is a question at this point in the succession, and that Arkell (1933; 1947), as shown in the upper diagram, correlated the base of the Cherty Series of Purbeck with that on Portland.

Glaucolithites glaucolithus Zone.
Index species: Glaucolithites glaucolithus Buckman 1923
The base of the Zone is defined as the first appearance of the species of the genus Glaucolithites. The type locality for the Zone is Hounstout Cliff, near Chapmans Pool, Isle of Purbeck. The White Cementstone with G. caementarius fixes the base of the Zone according to Wimbledon and Cope (1978). On Portland the strata corresponding to this are the lower part of the West Weare Dolomites ("West Weare Sandstones") above the Exogyra Beds.

Progalbanites albani Zone.
Index species: Progalbanites albani Arkell 1935
Wimbledon and Cope (1978) defined the base of the Zone as the first occurrence of the genus Progalbanites. The type section is at Hounstout Cliff, near Chapmans Pool, Isle of Purbeck. The Massive Bed is the lowest horizon with Progalbanites. On the Isle of Portland it is the Black Nore Sandstone.

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PORTLAND, PART 2 - GENERAL GEOLOGY continued
2.3 Geological Map and Cross-Section of Portland

Left: Simplified geological map of the Isle of Portland based on House (1993), the Ordnance Survey and Geological Survey maps and other sources. Note that quarrying has in some areas resulted in the removal of the Purbeck strata and a change in the geology of the area. Several quarries and areas of landslides are shown. PHH - Portland Heights Hotel with fossil trees and ammonites on display in the garden. PC area of celestite and the Portland Alabaster in the Soft Cockle Member of the Purbeck Formation. For detailed information please refer to the Ordnance Survey Map 1:50,000 Dorchester Sheet No. 194 or, better, the 1: 25,000 or 1:10,000. For geology please see the British Geological Survey 1:50,000 Weymouth Sheet 342 or for the adjacent sea-floor the 1:250,000 Portland Sheet 50N 04W, New Series including the Continental Shelf.

Right: 1884 geological map of the Isle of Portland, showing the Purbeck Formation extending further north, before the northern quarries had removed so much of this formation and the underlying Portland Stone.

Left:This is a simplified geological cross-section throught the Isle of Portland from north to south. It is diagrammatic and has a vertical scale exaggerated by three times the horizontal scale. For the sake of simplicity towns and some other non-geological features have been omitted (and it is, of course, not just a rock with a prison!). The real topography is more subdued than this and the strata dip southward at the low angle of about one and half degrees. These southward-dipping strata form part of the south limb of the Weymouth Anticline. They descend under the sea to the Shamble Syncline, south of Portland Bill. Note how the Portland Stone forms a prominant hill and scarp in the north overlooking Portland Harbour and the Chesil Beach. In the south it descends to just above sea-level at Portland Bill. Here it was easily accessible by sea and quarrying for the London churches after the Great Fire of London started here. Later the stone was quarried from the higher part of the " island " and in the north the Admiralty Quarries provided much material for breakwaters of Portland Harbour. The old prison nearby, which you will recognise from the high ground of Portland by its conspicuous chimneys, provided the labour force. The Kimmeridge Clay is a very thick unit which underlies the Isle of Portland and is the cause of the many landslides around the high cliffs of the northern part of Portland. This clay also underlies the Chesil Beach and Portland Harbour. The oil-shale was once visible at Castletown in the north and has burnt naturally in the past (at a time of lower sea-level). Red debris, including thermally metamorphosed shale, can still be found on the shores of Portland Harbour.
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Left: The rugged West Cliffs of Portland seen from near end of the Chesil Beach show the Portland Cherty Series above landslipped cliffs of Portland Sand. This provides a mirror image of part of the cross-section above. On these cliffs there are weares or tips of quarry debris, mostly from the early part of the last century. Fallen blocks of Portland stone litter the shore but the underlying Kimmeridge Clay at the base is exposed here and there and is slowly but progressively retreating on the very exposed and storm-beaten coasts.

Right: The same photograph with labels. In these cliffs only part of the Portland Stone is now visible. The Cherty Series, the lower part is well-exposed as the rock in the upper part of the cliffs but the overlying oolite of the Portland Freestone has been mostly quarried away in the early part of the Nineteenth Century. The Purbeck Formation which was here an overburden has also been largely removed. Inland from the cliff stop is the Portland Sculture Park in the old Tout Quarries. Sections of the Portland Freestone and the basal Purbeck strata with the Great Dirt Bed are still clearly visible there. Major landslides in the West Cliffs involve the Portland Stone and the Portland Sand but are based on the Kimmeridge Clay beneath. Green Ump is the result of an impressive series of rotational landslips which have turned the Purbeck strata vertical. Alleleuia Bay has the remarkable feature of the paved footpath disappearing beneath the beach (Alleleuia or Halleleuia Bay was named by the very religious personality Nuncle Hiram who once maintained moral standards on Portland by such action as beating a sailor senseless outside the Alexander Hotel for profaning! See Mackenzie, 1993).

PORTLAND, - GENERAL GEOLOGY continued

Offshore Geology

The geology of the seafloor around Portland was investigated by Donovan and Stride (1961) by an acoustic survey supported by coring, the observations of divers and sample collection. This map is based on their work with some minor modifications (the geology of the offshore area is also shown on the British Geological Survey 1:250,000 Sheet 50N 04W, solid geology).

The major offshore feature is the Shambles Syncline. This plunges towards the east. Note how the southern tip of Portland, Portland Bill, is positioned on the axis. This is unlikely to be a coincidence. The progresssive easterly swing of the dip from the north to the south of the island, as noticed by Strahan (1898), is because the northern part of the Island is situated clearly on the north limb whereas Portland Bill on the axis is an area with a dip that is controlled by the low angle of plunge of the axis to the east.

The Kimmeridge Clay occupies a considerable area of the seafloor and Portland Harbour lies over these easily eroded material. West of the Chesil Beach ichthyosaur and other vertebrate remains from the Kimmeridge Clay have been found on the seafloor by divers.

The Portland Stone forms prominant ridges on the seafloor but this and the Purbeck Formation proved difficult to core. Ostracod limestones of Purbeck type were found in the eastern part of the submarine Purbeck outcrop shown on the map. The middle Purbeck limestones form a slight ridge on the seafloor, just east of the area shown on the map (Donovan and Strike, 1961).

What is shown as Wealden is not firmly proven to be such but it is very likely that it is. Donovan and Stride cored grey and faintly pink marl with lignite and pebbles of Purbeck limestone at one site but the material was disturbed and contained no diagnostic microfauna. Grey sand with carbonaceous matter was obtained and further east typical Wealden mottled clays and marls were found.

The Shambles Bank is a pronounced ridge rising about 20m above the seafloor. Donovan and Stride (1961) considered it to be a sandbank because Admiralty Charts and surveys record only sand, shingle and broken shells on its surface, and there are sand waves along the southern side. Two ridges at the eastern end are oriented differently from the remainder of the bank. The southern and larger one is assumed to be Portland Stone , whilst the northern one is a resistant bed in the Portland Sand.

Pingree (1978) has discussed the tidal origin of the Shambles Bank. More recent work has been done by Bastos, Kenyon and Collins (2000) and is still in progress. They have used bathymetric, hydrodynamic and side-scan sonar to investigate the sediment processes associated with the headland of the Isle of Portland. They have produced maps which show a fairly symmetrical pattern of submerged sand banks on both sides of the peninsula with erosion taking place at Portland Bill. Of the sand banks on the eastern side, the Shambles Bank, is in much shallower water than the western ones, and is therefore more well-known. There is greater influence of waves on the exposed west side of the headland and this produces some asymmetry of pattern.

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PORTLAND, - GENERAL GEOLOGY continued
Portland Stone Facies - Sedimentary Environments

(See also Portland Stone details, below)

The Portland Stone consists of carbonate-cemented lime-sands formed in shoal conditions at almost the end of the Jurassic Period. The shallowest conditions were during deposition of the uppermost beds, just before transition into the lagoonal Purbeck facies.

A broad view reveals a general regressive (shallowing) sequence from the relatively deep-water Kimmeridge Clay, sometimes almost anaerobic, through the shallowing Portland Sand. In spit of the name much of this is marl, but with influx of quartz sand and silt in the Black Nore Sandstone on Portland and the Massive Bed in Houns-Tout Cliff, Isle of Purbeck. Evidence of shallowing generally continues up into the Portland Cherty Series (Dungy Head and Dancing Ledge Members of the Portland Limestone Formation of Townson, 1975)). Considering it in more detail, there was a limited transgression (deepening) at the base of the upper part of the Cherty Series (Dancing Ledge Member of Townson) before the continued final shallowing.

The upper and final, Portlandian regression followed this transgressive episode and deposited the sponge-spicule rich lime muds of the Upper Cherty Series (Dancing Ledge Member). These coarsen up into shelly packstones with large epifaunal bivalves and sponges (epifaunal means living on the surface of the sea-floor, as opposed to living within the sediment). Continued shallowing brought in shoal water ooid grainstones (i.e. white oolitic lime sand) of the Portland Freestone or Winspit Member (Townson, 1975; Bosence, 1987). There were subaqueous dunes of white carbonate sand on the sea-floor. These were low-angle features and they were moved southward under the influence of currents or waves. Most of the movement took place during hurricanes and storms. In contrast during the quiet phases, which occupied most of the time, thick-shelled bivalves, sponges and shrimp-like crustaceans colonised these mounds.

PORTLAND, PART 2 - GENERAL GEOLOGY continued
2.6 The Portland Cherty Series

The Portland Cherty Series is well exposed in the cliffs around the Isle of Portland and also in the deeper quarries. Shown here is an example in the quarrying relic of Nicodemus Knob in northeast Portland, near the Verne Fort; this consists entirely of Portland Cherty Series.

PORTLAND PART 2 - GENERAL GEOLOGY continued
2.7 The Portland Freestone and its Facies

The Portland Stone is the uppermost part of the Upper Jurassic Portland Group and is the result of a shallowing of the late Jurassic muddy seas to give way to clean white shoals of lime sand under shallow, clear, blue-green waters in a warm climate. There were many molluscs but few corals on the sea-floor, but the occasional plesiosaur came in to the shallows to feed on the rather thick-scaled fish . The lithification of this carbonate sand produced the Portland Stone, which is mostly a porous oolite or oosparite, with only a limited amount of calcite cement between the allochems (grains). It contains many thick-shelled marine molluscs in places and here and there, remains of giant ammonites (Titanites). Most of the shells of organisms which consisted of aragonite (mother of pearl) have been dissolved leaving moulds. Oysters, with their calcite shells, remain almost unchanged.

There are two main beds quarried. The lowest is the Base Bed (which also has been known as the Best Bed, Lower Tier, White Bed or Bottom Bed - Howe, 1910). The Base Bed is 1.5 to 5.8 m in thickness. Above it is some cherty limestone - the Curf and Chert, 1.2 to 2.7m thick, which is poor stone and not quarried. It is absent in a quarry south of Weston. Over this is the main quarried bed of Portland Stone, the Whit Bed, a particularly good oolite. It is 1.2 to 4.6 m in thickness, but the thickest developments of the bed are seldom good throughout (Howe, 1910). This, (also once known as the White Bed - Fitton, 1836) is of a warm cream to pale brown tint when fresh, but becomes white on exposure (Howe, 1910). At the top is the porous shelly Roach (in Fitton, 1836, this is spelt "Roche"). A typical vertical succession through the Portland Stone with overlying basal Purbeck strata in a quarry on the northern part of the Isle of Portland is given in the diagram alongside (For more on Withies Croft Wall see Withies Web Page. ). There are differences in detail from quarry to quarry and individual beds may change a little in thickness, but, in general, this is a good representation of the Portland and basal Purbeck succession.

PORTLAND PART 2 - GENERAL GEOLOGY continued
Lateral Facies Relationships

The origin of the facies of the Portland Stone is explained in this classic schematic diagram of Townson (1975). This very useful contribution to the understanding of the Portland Group shows, for example, the relative position of th ooid shoal that gave rise to the Whit Bed. Walther's Law (a widely used principle of modelling vertical changes in lithology in terms of lateral changes in environment) has been used to produce this model. It is not necessarily the case that all these facies were developed to a near equal extent at any one time. The basal Purbeck facies shown here covered almost the whole region at one stage. Nevertheless, this is a good method of understanding the general origin of the facies.

Portland Facies - Revised Version of the Facies Diagram

The above diagram is a simplified and modified version of the classic diagram of Portland facies by Townson. It is not greatly changed, but is simplified a little, and incorporates some individual views of the present writer. Refer, in addition to the original for more detail on the facies shown, and read the key paper by Townson (1975).

PORTLAND, - GENERAL GEOLOGY continued
Jurassic Oolites - Introduction

The Portland Freestone is mostly oolitic on the Isle of Portland, but there are parts, particularly towards Portland Bill, where the ooids are less obvious. The name oolite comes from the Greek for "roe-stone" because it resembles fish roe. The ooids are visible with a hand-lens or stereoscopic binocular microscope, and examples of Jurassic ooids are shown in the examples above.

To illustrate the appearance of ooids at high magnification an SEM (scanning electron microscopy) image of another Jurassic oolite, from the older Corallian of Osmington Oolite is also shown. The Portland ooids are similar to this and, although now all calcite, they probably originally consisted of aragonite. They usually have concentric structure and have been formed in strong wave or current action on a shallow sea-floor in warm, carbonate-saturated sea-water.

Generally it is easy to see the Portland ooids with the aid of a hand-lens. If there is any difficulty then try different surfaces such as a weathered joint surface or a clean break. Sometimes a wet surface or one which has been lightly etched with dilute hydrochloric acid will show such the fabric of the rock well. They are coarse-grained and most conspicuous in the Roach Bed at the top.

GENERAL GEOLOGY

Portland Stone - Petrography

Before discussing oolite petrography the basic classification of limestones is considered briefly. Most geologists probably use the well-known Folk's Classification, although some, particularly in the oil industry, prefer Dunham's Classification. A simplified version of Robert Folk's (1962) scheme is given here. It is only satisfactorily used with good thin-sections, but with experience you can often make an intelligent guess at the Folk's name using a hand-lens (the diagram has the addition of grumeleuse micrite so that it is convenient to use when working on Purbeck limestones which may contain this).

Note that oolites are simply divided into oosparites with a sparry cement and oomicrites with a micrite matrix. The Portland oolites are dominantly porous oosparites. Oomicrites occur in the Corallian Osmington Oolite, lower in the Upper Jurassic.

The Portland oosparites, discussed below, have, for various reasons, less well-preserved ooids than those of the slightly older Corallian strata of Osmington Mills and elsewhere in the Weymouth area. The Corallian oolite are in excellent condition and are shown here as an ideal model of Jurassic oosparites. Compare and contrast with the Portland oolite thin-sections shown below.

It is important to note, as can be seen above (using potassium ferricyanide stain) that the cement of the Corallian oolite in the lower image is ferroan calcite (blue), whereas that of the Portland oolite seems to be non-ferroan.

The thin-section of Portland oolite, shown above, are both from the Whit Bed of the Portland Freestone of the east coast of the Isle of Portland. The uppermost shows the typical concentric-micritic ooids of the Portland Stone. They are obviously not the radial-calcite type, but it is not clear as to whether the original mineral was micritic or tangential aragonite or whether it was high-Mg calcite.

Notice that the varied nuclei. There are scattered quartz sand nuclei, and the interesting occurrence of bryozoan nuclei. You can see two examples in the thin section. The porosity has been filled with coloured resin. This brings out clearly the euhedral character of the cement. Large calcitic rhombs are visible as a rather coarse cement.

The lower two images are of the same thin-section; one labelled and the other not. This is from the same bed, the Whit Bed, at Sandholes, further south (it has been figured in an undergraduate project by Nicola Bucknell). It is, again, an oosparite with high porosity. The oolite represents the product of a well-developed and persistant type of carbonate shoal environment in this area at the end of the Jurassic Period. Influx of clay and iron had reduced to almost zero in this region during the Late Jurassic regression. The oolite is very clean. Notice that this thin-section shows a significant proportion of grapestone. These are aggregations of ooids that at the present day in the Bahamas tend to occur in the more marginal (seaward) facies of the ooid shoal.

The limestone is now almost entirely low-magnesian calcite. The fauna shows that it originated in shallow marine conditions, and is unusual for Jurassic limestones only in the rarity or absence of brachiopods. The petrography can now be considered further. Firstly, we will consider the ooids. We must compare with ooids in modern, shallow marine environments. The ooids are similar in size to modern examples and similarly are constituents of a clean, well-washed carbonate sand (i.e. there is no appreciable micritic matrix or clay). The main difference, though is that modern ooids are usually of aragonite, of needles with tangential orientation and with much organic matter. Aragonite is unstable in meteoric (rain-derived) water and is often either just dissolved or replaced by calcite. It is not surprising that the ooids are not aragonite. The question arises, though, as to just when the inversion of aragonite to low-Mg calcite took place. Now the Isle of Portland (especially the northern part) is on a Late-Jurassic to Early-Cretaceous structural high (Late Kimmerian). This has been discussed by Townson (1975) and West (1975) . Since these ideas were first put forward more information has become available as a result of petroleum exploration, and the initial tentative maps of the high may need modification, but the basic principle stands undisputed. The structural high raises the possibility of early uplift and, therefore, in this initial exposure to the vadose zone, an early phase of aragonite-calcite inversion and associated lithification. The evidence for this is very good, although not quite as good as Arkell (1947) seems to have considered. He regarded the limestone pebbles in the Great Dirt Bed as derived from the Portland Stone, and this would have indeed proved early lithification. In fact, they are sometimes oolitic but from a "quiet-water" oolite of the Purbecks and not from the Portland. They show that the Hard Cap of the basal Purbecks was indeed early lithified, but do not give direct evidence on the Portland oolite. However, the fossil trees rooted in the fairly thin soils above the early limestones show that uplift was significantly above the saline water table. Some strong evidence regarding the Portland comes from the Roach, the shell-beach bed at the top of the Portland Stone. This is notable for its mouldic secondary porosity. That is to say that the aragonite shells of this shell bed, such as the well-known gastropod Aptyxiella portlandica, have lost their shells and are preserved only as moulds. Shells of oyster-type bivalves, which are of calcite, are preserved with little alteration. Such mouldic secondary porosity is, of course, a classic feature of early vadose, selective dissolution. Early uplift, the evidence of which is clearly shown by the localised "fossil forests", is an obvious explanation.

The argument so-far seems straighforward. However there is a major problem. The selectivity of the mouldic secondary porosity process will operate very satisfactorily if the ooids were of calcite. By comparison with the modern examples we expect the ooids to have been aragonite initially (at this point look up the interesting publications of Maurice Tucker and see whether they really should have been aragonite at the time in the Mesozoic). Two working hypotheses are needed to advance further. A first hypothesis is that the ooids were of high-Mg calcite retaining the general fabric but losing Mg by incongruent dissolution. A second hypothesis is that the ooids were of aragonite and have undergone inversion before the dissolution of aragonite shells.

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Hypothesis One. What is the evidence supporting the calcite origin?

(i) There is no obvious neomorphic spar fabric replacing the ooids, but it might not be obvious in such small objects. We need to look at the ooids more closely and the image here does not provide enough detail. We really need scanning electron microscopy.

(ii) We use Occam's Razor and avoid the problem of two early stages of aragonite-calcite inversion.

Hypothesis Two What is the evidence supporting an aragonitic origin?

(i) Comparison with modern ooids. They are similar in their general features, including the dominantly concentric lamination and the occurrence of grapestones.

(ii) Not much else at present. We could examine the Sr content but it is likely to be in the usual range that is round about 350 ppm, that is typical for Jurassic calcite showing shallow rather than deep-burial diagenesis. Of course, it would be very interesting if a figure of near a 1000ppm was retained but this is unlikely. Sr isotopes might be useful.

.

Thus, there is no without definate evidence for one or other of the hypotheses. Further clues may exist, though, and we need to examine silificied ooids in chert for evidence. The silicification process at ooid/bivalve contacts needs study. Silicification was probably pH-controlled and is known to be early (note that solubility of silica is high at ph 9.4 and this attainable in limestones with interstitial water at low pCO ₂ ) . Portland chert is reworked into the Upper Purbeck Formation so it had formed as a replacement product (although perhaps still as opal or opal-CT) by the late Berriasian. Most of it is pre-compaction (or at least before major compaction), although it should be noted that there is some late chert which must not be confused with the early silica. There is no reworked chert in the Purbeck dirt beds (palaeosols) but erosion may not have penetrated far enough, so this is not a good proof of the absence of chert at this time. Careful consideration of the silicification may yield an advance. Try to make some progress on this, if you can. With the possibility of either hypothesis being correct we cannot advance further at the moment, but if the ooids were aragonitic, then there was a very early ooid aragonite/calcite inversion that preceded the meteoric water attack on the organic (bivalve and gastropod) aragonite.

Another aspect that also needs further investigation is the problem of lowest "palaeopiezometric head", in other words the lowest level of the water-table within the Portland Stone in Berriasian (i.e. Purbeck) times. The Basal Shell Bed does not seem to have been affected by the same type of processes as has the Roach shell-bed, and I would expect the water-table to have been above it. The comparison has probably not been studied in detail, though. Attention should be drawn to the work of Townson who has written a sedimentological thesis on the Portland Stone, and published some of this in a classic paper - Townson (1975).

There are other aspects of the petrography to be discussed, but I intend to return to them later. In the meantime think about the relatively high porosity, as mentioned on the image. There is celestite in the Portland Freestone at just a few places (e.g. celestite-replaced bivalves at Worbarrow Tout) showing some downward movement of sulphate. It is very limited though. There was a little halite in the Portland at Bacon Hole (some pseudomorphs at the top of the Portland). These occurrences bring up the possibility of a second, later evaporite cement on the euhedral, calcite (LMC) cement that is visible in the thin-section above. This could have been dissolved subsequently so that the apparent high primary porosity seen now is actually secondary. There are several reasons, some of which you may think of, as to why this is unlikely. I suspect that we are seeing, in fact, a retained primary porosity within the limited calcitic (scalenohedral) cement. Of course hydrocarbons could have been involved in retardation of cemention, but there is no clear evidence of this. On balance it seems sensible to attribute the retention of early fabrics to the early date of vadose processes. The probable cause thus is the early lithification of the Portland Stone.

PORTLAND, - GENERAL GEOLOGY continued
Portland Freestone - Patch Reefs

Between the ooid lime sand dunes discussed above there was some development of patch reefs (i.e. small reefs). They were made of the oyster - Ostrea and the bivalve Plicatula,which like oysters cements itself to hard surfaces or other shells. There were also red algae of the genus Solenopora and bryozoans (sea-mats) involved in the construction of these small reefs. They contain cemented internal sediments and were bored by Lithophaga, a boring bivalve, by sponges and by worms (Bosence, 1987). Patch reefs up to 5m high and 10m across occur in the quarries in the centre of the Isle of Portland (Townson, 1975; Bosence, 1987). Small ones are present at Portland Bill (SY 680705). In the reefs the shells are vertically stacked and make up most of the framework (ie. the structure of the reef). They are encrusted by the bryozoan Hypororpora portlandica. In places the red alga Solenopora portlandia contributes to the reef framework. This alga shows some seasonal banding with small-celled summer layers visible under the microscope (Wright, 1985).

As well as being preserved in situ evidence of patch reefs can be exist as eroded and transported blocks in coarse shelly layers within the ooid grainstones (oolitic limestone), according to Bosence (1987).

GENERAL GEOLOGY continued
Portland Freestone - Cross-bedding

The oolitic Portland Stone often shows large scale cross-bedding. This is indication of the high-energy conditions in which it was deposited. Probably, much of it is cross-bedded like this but favourable weathering conditions are needed before it is clearly visible. Oolites require high energy wave or current conditions in a warm shallow seas to rotate the ooid grains at frequentl intervals. The left-hand image is of an exposure near Admiralty Quarries in the northern part of the Isle of Portland. The right-hand one is from south of Mutton Cove.

GENERAL GEOLOGY continued

Portland Freestone - The Roach Bed

Left: The uppermost part of the Portland Stone Formation of the Isle of Portland is formed by the " Roach " bed. This is a shelly oolitic limestone with moulds of aragonitic fossil shells, particularly the " the Portland Screw " ,a cerithid gastropod - Aptyxiella portlandica . This was a shallow water mollusc, which is confined almost entirely to the Isle of Portland, and occurring in the last relatively normal marine strata, before the Purbeck lagoonal formation was deposited. It was probably a browser of cyanobacteria (" blue-green algae") and perhaps it had some similarity in ecology to the cerithid gastropods common in sheltered shallows at the margins of the Arabian (Persian) Gulf. Those, however, are typically lagoonal whereas the thick-shelled molluscs associated with it seem to indicate a higher energy environment. Preceding the lagoonal deposits it was clearly a gastropod of shallow water and the Roach may represent a shelly beach accumulation. The aragonite may have been dissolved by meteoric (fresh) water at an early stage, as happens today when aragonitic sediments are exposed to the atmosphere.

Right: The same Roach image, artificially modified by computer, to show it as a shelly sediment before dissolution of the aragonite, with shells in relief on a cut surface.

This photograph shows the typical appearance of the Roach in northern Portland. The example is at Withies Croft Wall, since destroyed by quarrying. The turreted gastropods Aotyxiella portlandica is abundatn in this northern part but is rare in the south of the Island. It is extremely rare on the Dorset mainland.

Left: The examples shown above are of the smooth, almost polished surface of limestone blocks used for the sea-wall of the Cobb harbour at Lyme Regis and shown in this image. The Roach, unlike the Portland oolite beneath, is not much used for buildings but, as waste stone, is widely used for sea-walls and sea-defences like this.

Right: The Roach has been used on a particularly large scale for the construction of the Portland Harbour breakwaters in the 19th Century. The breakwaters were planned in 1844 to shelter ships of increasing size and to form a strong naval base as a defence against France. By 1855 major progress was being made and 560 men were employed on the breakwater. Stone from the Admiralty quarries on Tophill, worked by convict labour, was deposited at the rate of 10,000 tons per week. The first stage was complete by 1867. There were forts at each end of the two initial sections and at the Nothe (Weymouth). A new section was built from the north side between 1895 and 1903 by civil engineering contractors. Later, in attempt to attempt to prevent submarine incursions the old battleship HMS Hood was sunk in the south entrance, the one nearest to the camera in the photograph. She turned over and now rests on her gunwales. (See Mackenzie, 1993 and some of the other publications listed in the general books on Portland).

The fossil content of the Roach of northern Portland is interesting. Everywhere the notable and obvious features are the cerithid gastropod Aptyxiella portlandica and trigoniid bivalves both of which are common as moulds. Townson (1975) noted that the characteristic fossils of the Roach are Laevitrigonia ("Trigonia"), Isognomon, Aptyxiella and Solenopora (a red alga), in a current deposited grainstone matrix. Blake had earler (1880) reported that the Aptyxiella portlandica is confined (almost!) to this bed. Sowerbya dukei and rare Buccinum naticoides also appear to be peculiar to the Roach but are much rarer. Laevitrigonia gibbosa, Lucina portlandica, Camptonectes lamellosus, Ostrea expansa and Natica elegans it retains from earlier times. The large cockle,Protocardia dissimilis, appears to have died out at this level (but the small euryhaline cockle - Protocardia purbeckensis comes in the lagoonal Purbeck Formation above).

Cliff and cliff quarry exposures are discussed here, but see also the other webpages with location descriptions, given below.

Please go also to:

Portland - the Inland Quarries of Portland

Portland Bill

Portland - Mutton Cove

Portland- Withies Croft Quarry

LOCATION:

- Church Ope Cove (east coast)
(SY 697711)

LOCATION:
Freshwater Bay (east cliffs near Southwell)
(SY 692703)

The cliffs south of Freshwater Bay consist of Cherty Series in the lower part of the cliff and with massive oolitic Portland Freestone in the upper part. It has been much quarried to produce a broad terrace or ledge along the cliff.

Freshwater Bay, locally called Neddyfield (Bruce, 1989), owes its name to a freshwater spring which emerged at the bottom of the square vertical face in the cliff, called Red Door. The grass covered building on top, between Cheyne house and the cliff top, was the pumping station for supplying fresh water to the naval base via an intermediate pumping station adjacent to Folly Pier. Bruce (1989) commented that just south of Red Door, large quantities of rock have been pushed over the cliff where previously there was a sandy beach.

Freshwater Bay is a good place to study the Portland Stone. A track just south of the Cheyne Weare car park leaves the main road and head down towards the cliff. It is not a good place to park but it is possible for one or two cars at most. It not suitable for larger vehicles or many cars. The track gives access to an old cliff top quarry in basal Purbeck limestones (Caps) over Portland Stone. The Hard Cap (Top Cap) above the Lower Dirt Bed contains many tree holes where branches of Purbeck coniferous trees have been coated in stromatolitic (thrombolitic) limestone and have subsequently rotted. Here and the cliffs further south are some of the best places to see these features. The holes are at various angles and the growth of the "algal" (actually microbial or cyanophyte) mounds around the branches must have been very rapid, as was the submergence. The Great Dirt Bed is present but not well-exposed here and lagoonal limestones with ostracods occur above.

From the small cliff top quarry (which he refers to as Freshwater East, map reference SY 6911 7019, and which Jane Francis refers to as Freshwater North) Howard Falcon-Lang (1998) reported rhomboid fish scales and vertebrae in the Skull Cap and a faint impression of a dinosaur footprint in the Transition Bed (Earlier Jane Francis, 1996, had referred to a dinosaur footprint in the Transition Bed without giving a specific locality but perhaps it is this one). Falcon-Lang mentioned a personal communication from A. Cole that well-developed vadose cements are visible in thin-sections of the Transition Bed. He also commented on the occasional occurrence of pyncnodont fish bones in the Portland Freestone Series.

A steep path down the cliff takes you to Red Door, on the north of the bay. Around here is a complete section of the Cherty Series. The Basal Shell Bed is accessible about 200m to the northwest of Freshwater Bay. This has the serpulid worm Glomerula gordialis especially common and there is a level full of the large ammonite Glaucolithites (House, 1993). Because of the high vertical cliffs which are fractured, safety helmets can be useful here. The cliff should be watched for hazards, such as unstable, loose rock. Is is important not to hammer the chert of the Cherty Series because it can produce dangerous splinters.

LOCATION:

Freshwater Bay - continued - Hardground

Damon (1884) recorded evidence for a Portland oolite hardground and for its penecontemporaneous erosion. Lithodomi (Lithophagus) borings have been filled with carbonate sediment, then penecontemporaneously lithified, before being eroded out with the harder fill projecting. These strange projecting casts were then coated with a thin layer of the coral Isastrea oblonga. Damon's record was for Grove Quarries (Red Croft) where there no longer a good exposure. However Lithodomi borings into shells of Camptonectes lamellosus can be seen very easily on waste slabs in Godnor Quarry, just inland from Freshwater Bay. If you walk in from the cliff for about 20 metres, you will see these on your right. This is probably the remains of a Portland Stone hardground. It may well be from the Curf, but I have not tried to locate the exact position of the waste slabs.

Early lithification of the Portland Stone, as noted elsewhere in this webpage, explains the lack of compaction in Portland ammonites and paired bivalves, but much of that lithification may have taken place in initial Purbeck times. The Roach has mouldic secondary porosity from the early access of meteoric water. This is explained by the end-Jurassic uplift to form the Purbeck island with the palaeosols and fossil forests which lie above. This is a different process from submarine hardground lithification but both can be the result of shallow water environments, and probably slow sedimentation.

The Portland oolite obviously originated in very shallow conditions and thus the sea-floor was prone to be drastically affected by small sea-level changes. The Curf hardground represents a mid-Portland Freestone event and there has clearly been an oscillation or oscillations of relative sea-level at this time. Note that the chert in the Curf and Chert probably indicates a slight deepening to Portland Cherty Series, sponge facies; the matter was not simple, movement was probably down as well as up (see also West (1975) for evidence of oscillating lagoon floor movement in the early Purbecks of Bacon Hole, near Lulworth Cove.)

LOCATION:

Freshwater continued - Great Dirt Bed of the Purbeck Formation

The basal Purbeck strata can be seen in the upper cliffs of quarries in the Freshwater Bay area. The sequence continues up to at least the Hard Slatt (e.g. well seen in Godnor or Sheats Quarry), the approximate level of the Jurassic/Cretaceous boundary. The basal Purbecks show some unusual features. The Great Dirt Bed, the famous rendzina palaeosol, changes character here from its pebbly form to the north into a southern facies of dark, carbonaceous shale without obvious pebbles. Near the border between these two facies at the high cliff quarry on southern side of Freshwater Bay a peculiar limestone boulder facies is present for a short stretch. This is present in a channel-like feature of unknown origin. Comparison should be made to the Great Dirt Bed at Worbarrow Tout. Both localities represent the deepening margin of the shelf or high facies of the basal Purbecks. The Great Dirt Bed has large clasts at Worbarrow but the section is complicated because it has also been involved in tectonic brecciation.

LOCATION:

Freshwater Bay - Red Door, below Cheyne (Chene)

Red Door is a steel door to the old Government Waterworks at Chene (Cheyne) on the north side of Freshwater Bay. It was well-known to geologists because it is at the best section for the Basal Shell Bed of the Portland Stone. When I was a boy you could not enter the locked door. Now the rusty Red Door is fallen and half-buried under boulders on the beach. The door gives some access to a cave, shown above. This is probably part of the 200 feet (60 metres) of galleries which have been driven into the Basal Shell Bed. I have been in the cave but have not tried to explore the galleries (if they are accessible).

The reason for the stratigraphical location is that the Portland Clay lies beneath the Basal Shell Bed and is an aquiclude holding up a perched aquifer. The aquifer, consisting of the Basal Shell Bed and probably part of the Portland Cherty Series is, of course, south-dipping and this is its lowest point above sea-level. The Portland Clay is actually visible at the doorstep of Red Door. The water issues from a fissure in the gallery and the yield has been quite high at 50,000 gallons a day ( Strahan (1898). The joints in the cliff here run at about N15degreesE. The old Government Waterworks Building can be seen on the top of the cliff and is near Cheyne House. To view this go to Cheyne Weare car park and walk on the cliff path south. You will enter an old quarry which ends to the south almost at the Waterworks Building and high above Red Door.

(Take care not to confuse the Government Waterworks at Chene (Cheyne) with the Southwell Well of 1892. The latter was a shaft was sunk midway between the Southwell village and western cliff (note - not the eastern cliff) by the Portland Urban Sanitary Authority. It commenced at about 170ft (52 metres) above O.D. The shaft was 230ft. deep and thus below sea-level, and from the shaft a borehole was made for a further 40ft. Thus the terminal depth was at about 100ft. (30m.) below sea-level and at the base of the Portland Sands (here 127 feet, 3 inches in thickness). At 220ft. about 130ft. of galleries were driven east and west. A good inflow was only found in the west heading and the eastern was dry. The water stood at 172 and a half feet from the surface, that is nearly at sea-level, and yield was about 90,000 gallons a day. The water had a hardness of 103 degrees and was contaminated by decomposed sewage (information from Mr. J. L. Webster). It is interesting that water was not found at the base of the Portland Stone as at Chene (Red Door), and therefore the Portland Clay is either impersistant here or fractured by jointing or faulting. The account here is based on ( Strahan (1898) and on ( Whitaker and Edwards (1926).)

It is possible to continue from Red Door northward toward Cheyne Wear. There is much scrambling over huge rocks. There is no path up at Cheyne Weare, and but the rock debris slope seems an easy matter for a climbers. Others, like me, may prefer to be roped up for safety, at least in the steep middle part.

LOCATION:

Freshwater Bay - Cheyne - Red Door - Basal Shell Bed

The Basal Shell Bed is a special feature of the Red Door area. It is the basal bed of the Galbanites kerberus Zone, according to Wimbledon and Cope (1978). The stratum conists of a grey, rather argillaceous limestone, with very little chert compared to overlying Cherty Series.

An illustration above shows what seem to be Lima ("Ctenoides") cunningtoni Cox 1928, although I have made no real study of this. Cox (1928) wrote:

"This peculiar species has long been exhibited in the Dorset County Museum, bearing the M.S. name of W.H. Hudleston's which is here adopted.; it was dedicated to Mr. William Cunnington of Devises, who made extensive collections from the Portland rocks of that district about eighty years ago. Its short anterior margin gives it a very distinctive appearance, and it is not closely comparable to any other known Jurassic species. It is referred to "Ctenoides" in the sense of authors, although there are certain nomenclatural objections to this name, which cannot here be discussed."

LOCATIONS:

Freshwater continued - South to Breston Quarries

LOCATION:

- Mutton Cove (West Cliffs)
SY 679712

Please go to the separate Mutton Cove to Wallsend Website, with information on gypsum and celestite in the Lower Purbeck Soft Cockle Member.

LOCATIONS

- Sandholes (east coast, south of Freshwater Bay and Breston Quarries)
(SY 688695)

Sandholes is, of course, the classic locality for studying "sand holes". This is the old quarrying term for the tree holes, where coniferous tree trunks have rotted away after heavy coating with hypersaline thrombolitic carbonate (stromatolites). The photograph shows the development of a small reef over a group of tree trunks. The sand holes are, as usual, in the Hard Cap (Top Cap) and the reef extends above the level of the mid Hard Cap dirt bed (poorly-developed palaeosol). The Great Dirt Bed, the main palaeosol, is at the top of the Hard Cap, just under the laminated limestone. The tree moulds are, as normal, above the Lower Dirt Bed, the palaeosol in which they grew. They are all at low angles, presumably due to the action of the saline flood. Notice that to the right there was a pair of tree trunks.

LOCATION:

- Southwell Landslip
(SY 695705)

The east coast of Portland differs from the west coast by displaying a long stretch of cliffs with topples in the lower part. This is Portland Stone capped by the basal Purbeck strata which has toppled seawards over the relatively incompetent Portland Sand (much of this is marl) and the Kimmeridge Clay beneath. This toppling may have happened at a time of lower sea level. Seen here from Cheyne Weare (a car park on the cliff top at the site of an old quarry tip) are large sloping slabs of Portland Stone. This is not of tectonic origin and the strata seen at the cliff top to the left are dipping at a low angle and are not far from horizontal (actually there is a gentle dip southward).

EXPOSURE LOCATIONS:
- Verne Fort

The Verne Citadel, now the Verne Prison, was built on the northern summit hill of the Isle of Portland, changing it greatly and destroying many archaeological features. We see it here, looking north from Verne Yeats car park (map reference - SY 691 732, east of the Portland Heights Hotel on the hill top. This is a good viewpoint and a good place to see the Chesil Beach too. The fort has a substantial moat, known as the "Verne Ditch" around its southern side. This was excavated in the last century. The position of the ditch can be seen as a horizontal line on the slope across the Tillycoombe Valley.

The geology of this area is interesting because here is the northernmost outcrop of Portland Stone on the Isle of Portland and this unit changes in its details from south to north. Damon, in 1884 described a section 75 feet (23m.) deep consisting of:

Soil
"Rubble and Cap" (i.e. Purbeck) - 20 feet (6m)
"Roach" - 14 feet (4.3m) (but not really Roach! - see below)
Impure limestone with chert ("flint") - 35 feet (10.7m) to 40 feet (12m)

This section is unusual with an apparently excessively large thickness of what Damon thought was "Roach". It is not easy to investigate the ditch now because it is part of the Prison. The Portland Screw Aptyxiella portlandica, so characteristic of the Roach, is almost entirely absent, while Perna mytiloides is numerous. Blake (1880), however, described a Trigonia Bed in this area beneath the Base Bed, although this is not usually mentioned in the accounts of other sections. It is just above the top of the Cherty Series and is of very variable thickness. In the northern part of the Island (exact locality not known) it was found to be 4ft (1.2m) to 6ft (1.8m) thick but at the Verne Ditch cuts down irregularly into the Cherty Series below, attaining a thickness of nearly 20ft (6m). Blake believed that it was this that Damon confused with the Roach and that, in fact, it is a shell bed at the base of the Portland Freestone Formation containing abundant Perna mytiloides. What is the explanation of this shell bed? Was a channel eroded during a temporary shallowing of the Portland sea and this channel then filled with a patch reef like those discussed below?.

The whole series at the Verne was described as being much shattered, and yielding no good building stone, although it furnished material for the construction of the Portland Breakwater. What the reason for the shattering? Was there a fault nearby or, more likely, has there been some incipient landsliding or opening of joints in this narrow neck of Portland Stone south of the Verne? Alternatively, has there been some cryoturbation (freezing-thawing action) during the Pleistocene as there has at Portland Bill (Pugh and Shearman, 1967) but on a larger scale?

The following fossils have been obtained from the Verne district. They are listed in their old names, as given by Damon, with some alternatives in more modern terminology.
.
Saurian remains. The well-preserved portion of a skeleton of a Plesiosaurus from the Verne area was displayed in the Museum of the Royal Engineers' Office at the Vern works in 1884.
Crustacea
Ammonites giganteus (Titanites anguiformis)
Ammonites biplex (Glaucolithites?)
Buccinum
Cardium dissimile (Protocardia dissimilis)
Cyprina
Lithodomus portlandicus
Modiola pallida
Ostrea expansa
Perna mytiloides
Trigonia gibbosa (Laevitrigonia gibbosa)
Trigonia incurva
Unicardium
Isastraea oblonga
(The Isastraea occurs as very thin layers of coral covering casts of the rock-boring bivalve Lithodomus)

Sauroptygians, plesiosaurs and pliosaurs seem to have been fairly common in the Portland seas, more so in fact than Ichthyosaurs (which also lived here), to judge from the lists of Delair (1993). Nine occurrences are known from the Portland Stone of Isle of Portland, in contrast to only three occurrences of Ichthyosaurs. The most common of the Portland plesiosaurs was Colymbosaurus trochanterius although Muraenosaurus and Aff Cryptocleidus richardsoni have also been found on Portland (Delair, 1993). Pliosaurus brachdeirus has been identified from Portland. Specimens are preserved in the Portland Museum, the Dorset County Museum, Dorchester and the Natural History Museum in London. Although they certainly also lived above clay sea floors, perhaps these shallow seas with their white lime sands and their warm, clear turquoise waters were particularly favourable environments for the real "sea-monsters" of Portland!

PORTLAND, PART 4 - EXPOSURE LOCALITIES - QUARRIES AND CLIFFS
- Verne Fort continued - Archaeological Finds

Many archaeological remains were found on Verne Hill during the excavations for the fort. Captain Mascall of the Royal Engineers communicated a list to Damon who published it in his book (Damon, 1884, p. 240-242). (see Morris, 1990, Fig. 91, for a good photograph of Captain Mascall's construction work for the Verne Citadel taking place in 1877). Here are just a few of the more interesting items in the Captain's list

"Ball of chert or dark flint used a matrix for forming flint implements." Within the Entrenchment on the upper portion of the West slope of the Verne.

"Flint arrow heads etc. in an ancient "Midden" with large quantities of cockle and limpet shells, by the side of the road to Verne from Fortune's Well, in the top of the drift, near S.W. glacis. " Also - "boar's tusk and bone implements in Midden on Verne Road."
( My comments: Are these remains by the Verne Road in the Tillycombe Valley of Mesolithic age? Are they of about 9,000 years BP which is about the date for the Mesolithic? Sea-level in the English Channel was then much lower at this time, about 25 or 30m lower (Bell and Walker, 1992). What is now fairly shallow sea-floor east of Portland would then have been mostly land. The sea was close to the west side of Portland, however, and judging from the present submarine contours it may have been only 2 km to the west. Most of West Bay would have been in existence, it being an area of lower ground flooded early in the Flandrian Transgression. Was a precursor of the Chesil Beach in existence then? The limpets obviously could have come from the rocks of the Portland headland (then like St. Alban's Head is now) but where did the cockles (Cardium or Cerastoderma) come from? Was there an estuary? A further separate question is why did the ancient people live just here? Did the spring of Fortune's Well once rise higher up the valley )

"Part of a speculum or mirror, bronze gilt, found with bones in the S.E. glacis about 1878, near F8."

"Copper stirrups four or five feet beneath the surface." Top of Verne Hill. (My comment: When were stirrups invented? Copper seems a weak material to use for them! This, however, is not my field of study.)

"Spearhead found in debris of cliff, four feet below surface." East, between Verne Hill and the sea.

"Upper portion of skull found with skeleton much decayed, enclosed within a kist of stone slabs placed on edge, and covered with similar slabs." (Also other bones and coffin of an infant).
( My comment: Many Roman stone coffins were found under the council estate built in the 1950s on the west side of the Verne, according to Morris, 1990, caption of Fig. 56)

"Kimmeridge Clay shale used as the lining of a grave." Within the Entrenchment on the upper portion of the West Slope of the Verne.
(My comment: Only the Kimmeridge oil shale would be sufficiently resistant for use as a grave lining. This could have been brought from Kimmerdge or from the Ringstead area. There is, however, a source of Kimmeridge oil shale nearer at hand, at the base of the Verne Hill on the north side. It is almost at sea-level now although it has been worked in relatively recent times. In Roman times when sea-level was lower it would have been more conveniently accessible. )

"Disc of Kimmeridge Clay shale." South slope of the Verne Hill near flank of S.E. Demi-Bastion at top of Escarpment.
(My comment "Kimmeridge coal money" - discs resulting from the cutting of amulets from Kimmeridge oil-shale. )

"Gold coin, 97 grains, Gaulish type. Copied from a stater of Philip of Macedon. Obverse a" head of Apollo " ; reverse - Chariot commemorative of victory at Olympic games." Also - "Finger bone with a spiral ring of bronze." South slope of Verne Hill near the flank of the S.E. Demi-Bastion at the top of the Escarpment.

"Skull of an ox found within a circular dry stone wall, about five feet in height, the circle six feet in diameter; and other bones much decayed; also a large quantity of ashes." South slope of Verne Hill near the flank of the S.E. Demi-Bastion at the top of the Escarpment.

"Copper coins (Antonius) found near No.4, two feet below the surface." North Common, below Verne Hill.

"Coin, silver penny, Henry III, Terre le Chaunier, 1222."

(Other items, including pottery, a knife, iron shot, enamelled brooch, bronze ring, a circular stone, and other bones and skeletons. More than 50 items in all are listed in two and a half pages of text.)

EXPOSURE LOCATIONS:

- Wallsend to Mutton Cove

Please go to the separate Mutton Cove to Wallsend Website, with information on gypsum and celestite in the Lower Purbeck Soft Cockle Member.

LOCATION:

- West Weare (West Cliffs) - Greenump (Green Ump) and Clay Ope

Landslipped cliffs of West Weare seen from the promenade and sea wall at the southern end of the Chesil Beach. The rotational landslide of Greenump or Green Ump (i.e. green hump) is a small mound near the sea in the distance.

Here is a view of the southern end of Greenump showing the remarkable landward dip of the Portland Stone. This has resulted from the rotation process of landslipping. It may have been rotated progressively with a series of landslides. In the distance is another rotational slide at Blacknore.

A potential topple of Portland Cherty Series can be seen from the cliff top of the West Cliffs. It is separated from the main outcrop by a major joint or fissure and is slowly moving outward and downslope. A previous collapse of something similar to this has caused a false alarm of an earthquake at Chiswell. The topple is probably not as large as those in the past because the Portland Freestone has already been quarried of the top of this rock.

The large blocks of Portland Stone forming the Tar Rocks are joint-bounded and not appreciably worn or rounded. They are probably the remains of an old topple of the Portland Stone high in the cliff, like the one shown above. If this is their origin, then their collapse is earlier than the rotational slide of Greenump. The fall was unlikely to have been more recent than about a thousand years ago.

Clay Ope takes its name from a place with clay that is open, i.e. an embayment. It is situated between the West Weare Cliffs and Blacknore (or Black Nore). Argillaceous parts of the Portland Sands are exposed here in the cliffs, but the name may have originated from a surface or submarine exposure of the underlying Kimmeridge Clay.

You can walk towards Clay Ope by taking the landslide-disturbed footpath south from the promenade at Chiswell. The route is easy until the southern end of Greenump is reached near Tar Rocks. You will now encounter large boulders and rocks which have to scrambled over. Some are as large as a car and a little agility and balance is needed to cross them safely. In the cliff at the back of the embayment of Clay Ope there are good exposures of Portland Sands. These are probably the best on the Isle of Portland. Interesting comparison can be made with Houns-Tout Cliff at Chapmans Pool, near Kimmeridge, where similar strata are exposed.

PURBECK FORMATION:
- Dinosaur Footprints in the Purbeck Formation of Portland

Please go to separate webpage on Portland Dinosaur Footprints.

PURBECK FORMATION OF THE ISLE OF PORTLAND:
- Dirt Beds (Palaeosols)

See also:

Fossil Forest, Lulworth Cove.

The Great Dirt Bed was originally recognised as a fossil soil by Webster in 1826. It is generally a dark brown to black carbonaceous marl with limestone pebbles that was originally a rendzina soil (West, 1979). A proportion of the limestone pebbles are dark grey ("black pebbles", similar to the cailloux noire in the Purbecks of the Jura Mountains of France and Switzerland).

Much study of the Great Dirt bed on the Isle of Portland and Lulworth Cove has been undertaken by Francis (1986 and other papers). She noted that the silicified remains of trees indicate that the Pubeck conifers were single-trunked (monopodial) upright trees with low branches. The shallow roots which spread laterally through the soils, but did not penetrate the underlying limestone.

The top carbonaceous layer (A-horizon) of the Great Dirt Bed appears to have been dominantly formed of plant debris derived from the local vegetation, on a clay base (Francis, 1986). The main constituents were conifer twigs and foliage. These contained large quantities of resin (Francis, 1983) which was the probable source of orange resinous blobs found under the microscope. The undergrowth of this forest of resinous conifers was of ferns and lycopods, and their foliage would also have contributed to the soil debris. Grass or other flowering plants had not evolved at this date.

This dirt bed, like the others (the Basal Dirt Bed and the Lower Dirt Bed) was found by Francis (1986) to contain fusain or fossil charcoal, as shiny millimetre-size rectangular blocks. She noted that the quantities of fusain were small and that the tree stumps were not charred. Thus, fires in the forest were generally confined to the small plants of the forest floor.

PURBECK FORMATION OF THE ISLE OF PORTLAND continued

Fossil Trees

See also:

Fossil Forest, Lulworth Cove.

The large Purbeck, silicified tree shown above and below is on display at the Portland Heights Hotel. It shows the presence of tensional roots (like a modern Cypress-type tree) that helped to hold against some strong, late Jurassic winds. It may have been eventually blown over before submergence is the Purbeck hypersaline lagoon.

Probably the best Purbeck silicified tree trunk is the well-known one preserved and on display in the northern part of the island. As shown in these photographs this particular specimen is preserved on display, although not in situ, in the garden of the Portland Heights Hotel on the hilltop of Portland (a Miocene silicified tree in the Western Desert of Egypt is shown for comparison). The trees are normally rooted in the Great Dirt Bed with its pebbly soil and presumably that is where this once has come from (although elsewhere silicified coniferous trees are also found in the Lower Dirt Bed).

It would be good to know exactly where this tree came from, and whether it was found still standing upright or whether it had been blown over and was found lying flat. It was originally on display at the quarry company headquarters where there was once a fossil garden, on the left a few hundred metres down the road south to Easton (near the old Drill Hall). It had been kept in the fossil garden for many years and is likely to have come from the old quarrying in this northern part of Portland. The tree is rather compressed in one plane and it is not known whether it was lying flat or was still in upright position. Its original orientation on a vertical axis (i.e. which side was originally north) is not known.

It is a splendid silicified tree trunk and it shows some interesting details. The now silificified tree is very resistant. About 140 million years ago dinosaurs and rats once stood next to it. It is remarkable that it has survived in such good condition. The fibres are clearly visible. The trunk has some knots and the upper part of two eccentric and exposed roots are preserved in silicified condition. Where the roots are present within the soil, the Great Dirt Bed, they are usually not silicified and in friable carbonised condition. You can see on the present southwest side the are twisting of the fibres where the exposed root joins the trunk. On the east there is a large root shown in cross-section. This is not symmetrical but has the earlier growth nearer to the ground. The irregular development of roots is unusual in Purbeck trees and the reason is not known. The trees were growing in a soil of limited thickness on hard, already lithified limestone. Was the irregularity in some way related to the limestone bedrock or was it a response to severe winds. Irregular growth of conifers can be seen at present on limestone mountains, but sliding down slope and regrowth in a new orientation can be the cause in that case. Here the ancient Purbeck limestone topography was low and very subdued. Another feature of interest is that knots, where small side-branches joined the trunk, can be seen if you examine the trunk of the tree carefully. Away from the roots the trunk, like that of most Purbeck trees, is fairly straight and does not show much twisting. The tree is preserved mostly as chalcedony (not wood opal) and it weathers to light grey, almost white. Freshly cut material from the inside, however, is very dark, can be almost black and gives off a sulphurous and bituminous odour on cutting. Much more organic matter is retained in these trees than is obvious on the weathered exterior. Note that although the wood structure is well-shown there are patches of poor preservation and cracks and veins filled with coarser chalcedony or quartz. These imperfections impose limitations on the study of the tree-rings, which are seen in some parts but not all.

There is much information about these Purbeck silificied trees in an interesting paper by Professor Jane Francis (1983) on the " Dominant Conifer of the Jurassic Purbeck Formation ". She mentioned that the growth rings were usually about only 1mm wide. The Purbeck trees were long-lived, often more than 200 years, and the largest specimens, like this, may have lived more than 700 years. Protocupressinoxylon purbeckensis was a quite ordinary Cypress-like conifer and if you saw it growing in a park now you would not recognise it as unusual. It is not as primitive or as odd as the "monkey-puzzle " tree (Araucaria) which has a longer history. Some animals have changed but the trees have not changed much since the Mesozoic Era.

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Protocupressinoxylon - Taxonomy

Note that there has been recent taxonomic literature that concerns the taxonomy of the Purbeck trees ( Philippe and Bamford (2008). They state that the name Protocupressinoxylon is illegitimate:

"Protocupressinoxylon Eckhold (Eckhold, 1923: 491)- a nomenclatural synonym of Protobrachyoxylon Holden, 1913 (Philippe, 1993), but not of Paracupressinoxylon Holden ex Torrey (the mention of both syntypes of Paracupressinoxylon being included by Eckhold is an error of Vogellehner (1968: 152)). The name Protocupressinoxylon is illegitimate, and the circumscription of its legitimate nomenclatural synonym (Protobrachyoxylon) is doubtful (see this entry above). In xylological literature Protocupressinoxylon is used by most authors as the name of a morphogenus including woods with mixed type of radial pitting and cupressoid oculipores. Most if not all the Protocupressinoxyla described to date clearly have cupressoid oculipores, but arranged in araucarioid cross-fields, which is not in contradiction with Eckhold's diagnosis but which puts them close to Brachyoxylon Hollick et Jeffrey (Philippe, 2002). In the literature there is a great amount of confusion because although some authors give a clear definition for "cupressoid oculipore", none to our knowledge have drawn a clear line between "cupressoid cross-fields" and "araucarioid cross-fields" (see however IAWA, 2004). In our opinion all the ambiguity about the use of Protocupressinoxylon originates here. In 1995 Philippe made two proposals, firstly to consider "araucarioid" a cross-field with numerous oculipores (either cupressoid or taxodioid) which alternate and which are contiguous (note that the areola of these semi-areolate pits is frequently faint or even not preserved in fossil wood); and secondly that "cupressoid cross-fields" be considered as a cross-field with few (usually no more than four) cupressoid oculipores, widely spaced and usually ordered in horizontal lines or columns. However, the earlywood of modern Araucariaceae very rarely has the cupressoid type of cross-field, whereas modern Cupressaceae s.l. very rarely have exclusively araucarioid cross-fields in the earlywood (but again this is not intrinsically relevant to building a parataxonomy). Amongst Mesozoic wood species already described we cannot identify a clear and unambiguous candidate for a neotype for Protocupressinoxylon. Should such be identified, we think a proposal for the conservation of that genus ought to be put forward."

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Other fossil material from the quarries is on display here and excellent specimens can also be seen at the Museum on Portland. Silicified trees are still found from time to time in the Portland quarries and specimens can be seen in the old tips or Weares. Purbeck silicified trees have also been found from time to time in the Vale of Wardour. Mr. John Needham and Izzy Needham search for such remains and have a superb collection. An example of a large branching Purbeck tree is shown below.

This silicified tree was figured by Fitton in 1836 from a careful drawing by Sowerby. It is not a wide trunk but is notable for its preserved length - 7.2 metres. Notice the number and positions of knots. This specimen was found in Dungeness Quarry on the Isle of Portland (north-western part?). A similar tree trunk from the same quarry was preserved against a house wall on Portland and this too was undivided for about 5 metres. Branches were small in comparison with the main stem. Because the tree was found in a compressed flattened state, and because trees are not known to project a great height through the overlying strata, it presumably was lying flat, having been blown over by a storm (or possibly pushed over by a dinosaur)

Some dimensional data for Fitton's tree is tabulated below. Height is given from the extremity of the preserved root. Two diameters and a mean are given because the tree has been compressed when lying flat. The percentage compression figure is the percentage of the transverse diameter to the mean diameter.

Data on Fitton's Tree

At the Height of -		Greatest Diameter	Transverse Diameter	Mean Diameter	Percentage Compression
0.61m.		0.476m.	0.318m.	0.397m.	80.1%
1.22m		0.419m.	0.286m.	0.353m.	81.0%
3.35m.		0.375m.	0.292m.	0.334m.	87.4%
5.49m.		0.381m.	0.241m.	0.311m.	77.5%

Thus, the average compression is 81.5% which is quite limited and suggests that the tree trunk was not compacted under a great overburden. Silicification (and lithification of associated stromatolitic carbonate) was probably early. The narrowing upwards, expressed as a change in diameter is 0.018m (i.e. about 2 cm) per metre height for a trunk of about a third of metre wide. It would be interesting to know how this compares with existing Cypress or Juniper - type trees (any comment from a forestry specialist?).

Trees frequently stand about a metre in height above the Great Dirt Bed on Portland, and in some cases about 2 metres (note the one preserved on the top of Portland and shown in an image above), according to Fitton (1836). The 2 metres is probably some indication of the depth of hypersaline water following the submergence of the trees.

No roots penetrate the underlying limestone (the Hard Cap). Pebbles derived from this in the Great Dirt Bed, and also Tepee structures, show that it was already lithified before the soil was developed on top. In one case a prolonged branch of a root of a tree has been bent out of its course by meeting the limestone and was continued horizontally along the surface for several centimetres (Fitton, 1836).

Trees frequently stand about a metre in height above the Great Dirt Bed on Portland, and in some cases about 2 metres (note the one preserved on the top of Portland and shown in an image above), according to Fitton (1836). The 2 metres is probably some indication of the depth of hypersaline water following the submergence of the trees.

No roots penetrate the underlying limestone (the Hard Cap). Pebbles derived from this in the Great Dirt Bed, and also Tepee structures, show that it was already lithified before the soil was developed on top. In one case a prolonged branch of a root of a tree has been bent out of its course by meeting the limestone and was continued horizontally along the surface for several centimetres (Fitton, 1836).

Purbeck Trees - Comparisons

See also:

Fossil Forest, Lulworth Cove.

The silificied trees of the Purbeck have been shown by Jane Francis to have cheirolepidaceous conifers. Rather similar trees occur in the Wealden of the Isle of Wight. A recent paper by Axsmith has discussed Lower Cretaceous (Aptian/Albian) cheirolepidaceous conifers from Arkansas (this paper also gives the main Wealden references). It is interesting that the like the Purbeck trees these Arkansas also lived in a environment sufficiently arid for the presence of evaporites such as gypsum. The morphology of the trees was shown by the author, though, to have helical branching rather than whorl branching which characterised the common Wealden trees.

PURBECK FORMATION

Tree Holes of the Hard Cap (Top Cap)

See also:

Fossil Forest, Lulworth Cove.

Hollow moulds left by the decomposition of tree remains are very common in the Hard Cap. They are mostly inclined at low angles. They were known to the Portland quarrymen as "sand holes". The Hard Cap (or Top Cap) is beneath the Great Dirt Bed and the trees were presumably rooted in the Lower Dirt Bed. They are general most abundant within about a metre or two of this lower palaeosol. They are best seen in the Sandholes area, the name referring to these moulds. They are present further north and in other parts of Portland. Silicification of trees in the Haerd Cap (i.e. above the Lower Dirt Bed) is uncommon on Portland, although it is a feature of the Purbeck Caps at some places on the mainland.

Hollow moulds of trees are also common in the Soft Cap, but here there are generally larger and mostly of vertical orientation. In some case similar tree trunks are preserved in silificied condition.

At the eastern edge of Yeolands Quarry near the cliff top and employed as a type of marginal barrier are many large blocks of stromatolitic (thrombolitic) limestone from the Purbeck Caps. Some of this is of typical Hard Cap type. In this there tree holes or "sandholes" where timber was encased in stromatolitic limestone and the wood subsequently rotted. These Hard Cap tree holes are usually of small diameter and at low angles, i.e. nearly horizontal. Here as in the Southwell area the holes have, in many cases, a partial filling developed after the decomposition of the wood. These incomplete fillings may consist of pelletoidal limestone, common at this level, or stromatolitic limestone or both. The remarkable aspect is that some Purbeck "stromatolitic" limestone was formed in partial darkness. This accords with the presence of "stromatolitic" material in a cylinder around the near-horizonal trees, and is thus not only above, but also beneath. Typical stromatolites at the present day are formed by Blue-Green microrganisms that contain chlorophyll. Thus an origin in considerable shade if not total darkness is puzzling and requires further investigation.

PURBECK FORMATION

Purbeck Cycadophyte Trees

In addition to the tall Cypris-type trees of the Purbeck forests, there were also cycadophytes. These much resembled modern cycads, plants of rather palm-like appearance, although unrelated to the angiosperm palm trees. They occur mainly on the Isle of Portland, especially in the Lower Dirt Bed.

This image clicks to the Barry Marsh, Southampton University, website where museum specimens of silicified cycadophytes are shown. The specimens are most likely to have come from the basal Purbeck strata of the Isle of Portland.

Cycadophytes - Further Notes on the Plates by the Rev. William Buckland (1837)

[It is interesting to read that the preservation in silica of the Purbeck cycadophytes of Portland is so good that William Buckland was able to discover down or cotton surrounding the trunks (see notes below). I wonder whether the birds of the time (this is post-Archaeopteryx) used this fluffy material for their nests, just as birds today sometimes use, for their nests, fibres from the outer brown woolly material around palm trees trunks]

"Plates 60, Fig. 1, and 61, Fig. 1, represent very perfect specimens of fossil Cycadites from Portland, now in the Oxford Museum; both having the important character of Buds protruding from the Axillae of the leaf stalks.

The section given in Pl. 59, Fig. 2, of the trunk of a recent Zamia horrida, from the Cape of Good Hope, displays a structure similar to that in the section of the fossil Cycadites megalophyllus from the Isle of Portland; (Pl. 60, Fig. 2) each presents a single circle of radiating laminae of woody fibre, B, placed between a central mass of cellular tissue, A, and an exterior circle of the same tissue, C. Around the trunk, thus constituted of three parts, is placed a case or false bark, D, composed of the persistent bases of fallen leaves, and of abortive scales. The continuation of the same structure is seen at the summit of the stem, Pl. 60, Fig. 1, A. B. C. D.

The Cycadites microphyllus, Pl. 61, Fig. 1, affords a similar approach to the internal structure of the stem in the recent Cycas. The summit of this fossil exhibits a central mass of cellular tissue (A), surrounded by two circles of radiating woody plates, B. b., between these laminated circles, is a narrow circle of cellular tissue, whilst a broader circle of similar cellular tissue (C) is placed between the exterior laminated circle, (b) and the leaf scales (D). This alternation of radiating circles of wood with circles of cellular tissue, is similar to the two laminated circles near the base of a young stem of Cycas revoluta, (Pl. 59, Fig. 3.) This section was communicated to me by Mr. Brown early in 1828, to confirm the analogy which had been suggested from the external surface, between these fossils, and the recent Cycadere; and is figured in Geol. Trans. N. S. vo1. ii. Pl. 46.

In Pl. 61, Figs. 2, 3, represent two vertical sections of a Cycadites microphyllus from Portland, converted to Chalcedony. These slices are parallel to the axis of the trunk, and intersect transversely the persistent bases of -the Petioles or Leaf-stalks. In each rhomboidal Petiole, we see the remains of three systems of vegetable structure, of which magnified representations are given Pl. 62, Fig. 1, 2, 3. We have, first, the principal mass of cellular tissue (f); secondly, sections of gum vessels (h) irregularly dispersed through this cellular tissue; thirdly, bundles of vessels, (c), placed in a somewhat rhomboidal form, parallel to, and a little within, the integument of each petiole. These bundles of vessels are composed of vascular woody fibres proceeding from the trunk of the plant towards the leaf. See magnified section of one bundle at Pl. 62, Fig. 3, c'.

A similar arrangement of nearly all these parts exists in the transverse section of the leaf stalks of recent Cycadeae. In Cycas circinalis, and C. revoluta, and Zamia furfuracea, the bundles of vessels are placed as in our fossil, nearly parallel to the integument. In Zamia spiralis, and Z. horrida,. their disposition within the Petiole, is less regular, but the internal structure of each bundle is nearly the same. In Pl. 62, Fig. A shews the place of these bundles of vessels in a transverse section of the leaf stalk of Zamia spiralis; Fig. A. c'. is the magnified appearance of one of the bundles in this section; Fig. B. c" is the magnified transverse section of a similar bundle of vessels in the petiole of Zamia horrida. In this species the vascular fibres are smaller and more numerous than in Z. spiralis, and the opake lines less distinct. Both in recent and fossil Cycadeae the component vascular fibres of these bundles are in rows approximated so closely to each other, that their compressed edges give an appearance of opake lines between the rows of vascular fibres, (see Pl. 62, Fig. 1, c'. Fig. B, c". and Fig. 3, c'.) These bundles of vessels seem to partake of the laminated disposition of the woody circle within the trunk.

An agreement is found also in the longitudinal sections of the Petioles of recent and fossil Cycadeae. Pl. 62, Fig 1, is the longitudinal section of part of the base of a Petiole of Zamia spiralis, magnified to twice the natural size. It is made up of cellular tissue, (f), interspersed with gum vessels, and with long bundles of vascular fibres, (c) proceeding from the trunk towards the leaf. On the lower integument, (b') is a dense coating of minute curling filaments of down or cotton, (a) which being repeated on each scale, renders the congeries of scales surrounding the trunk, impervious to air and moisture.

A similar disposition is seen in the longitudinal section of the fossil Petiole of Cycadites microphyllus represented at Pl. 62, Fig. 2, and magnified four times. At f, we have cellular tissue interspersed with gum vessels, h. Beneath c, are longitudinal bundles of vessels; at b, is the integument; at a, a most beautiful petrifaction of the curling filaments of down or cotton, proceeding from the surface of this integument.

In the vascular bundles within the fossil Petioles, (c) Mr. Brown has recognized the presence of spiral, or scalariform vessels (Vasa scalariformia) such as are found in the Petioles of recent Cycadeae; he has also detected similar vessels, in the laminated circle within the trunk of the fossil Buds next to be described. The existence of vessels with discs peculiar to recent Cycadeae and Coniferae, such as have been described in speaking of fossil Coniferae, has not yet been ascertained.

This plant had been living many years, in Lord Grenville's conservatory at Dropmore. In the autumn of 1827, the external palt of the scales was cut away to get rid of insects: in the following spring the buds began to protrude. Similar buds appeared also in the same conservatory on a plant of the Zamia spiralis from New Holland. In vol. vi. p. 501, Horticult. Trans. leaves are stated to have protruded from the scales of a decayed trunk of Zamia horrida in a conservatory at Petersburgh. I learn from Professor Henslow, that the trunk of a Cycas revoluta, which in 1830 produced a Cone loaded with ripe drupae, in Earl Fitzwilliam's hothouse at Wentworth, threw out a number of buds, from the axillae of the leaf-scales soon after the Cone was cut off from its summit. In Linn. Trans. vol. vi. tab. 29, is a figure of a similar cone which bore fruit at Farnham Castle, 1799.

It is stated in Miller's Gardener's Dictionary, that the Cycas revoluta was introduced into England about 1758, by Captain Hutchinson; his ship was attacked, and the head of the plant shot off, but the stem being preserved, threw out several new heads, which were taken off, and produced as many plants.

In the fossil trunk of Cycadites microphyllus, Pl. 61, Fig. 1, we see fourteen Buds protruding from the axillre of the leaf stalks, and in Pl. 60, Fig. 1, we have three Buds in a similar position in Cycadites megalophyllus.

In Pl. 61, Figs. 2, 3, exhibit transverse sections of three Buds of Cycadites microphyllus. The section of the uppermost bud, Fig. 3, g, passes only through the leaf stalks near its crown. The section of the bud, Fig. 3. 'd, being lower down in the embryo trunk, exhibits a double woody circle, arranged in radiating plates, resembling the double woody circle in the mature trunk, Pl. 61, 1, B, b. But in Pl. 61, Fig. 2, the laminated circle within the embryo trunk near d, is less distinctly double, as might be expected in so young a state. At Pl. 62, Fig. 3, d, and d', we see magnified representations of a portion of the embryo circle within the Bud, Pl. 61. Fig. 3, 'd. These woody circles within the buds, are placed between an exterior circle of cellular tissue, interspersed with gum vessels, and a central mass of the same tissue, as in the mature stems.

On the right of the lower bud, Pl. 61, Fig. 3, above b, and in the magnified representation of the same at Pl. 62, Fig. 3, e, we have portions of a small, imperfect laminated circle. Similar imperfect circles occur also near the margin of the sections, Pl. 61, Figs. 2, 3, at e, e', e"; these may be imperfectly developed Buds, crowded like the small Buds near the base of the living Cycas, Pl. 58: or they may bave resulted from the confluence of the bundles of vessels, in the Bases of leaves, forced together by pressure, connected with a diminution or decay of their cellular substance. The normal position of these bundles of vessels is seen magnified in Pl. 62. Fig. 3. c. and in nearly all the Sections of Bases of petioles in Pl. 61, Fig. 2."

PURBECK FORMATION
- Purbeck Fossil Insects on Portland

Purbeck dragonfly nymphs have been found by Jarzembowski and Coram (1997) in the Lower Purbeck limestones of Freshwater Bay, Isle of Portland. They are also found in the " Cypris " Freestone Member of Durlston Bay. Evidence of hypersalinity, such as pseudomorphs after halite and gypsum, is common in the " Cypris Freestones " Freestones and this, in contrast, is interesting evidence of the existence of freshwater pools. With the dragonfly larvae are water bugs ("water boatmen") and dytiscid-like beetles and aquatic larvae of nematoceran flies. Freshwater insects can occur in association with hypersaline environments at the present day. Sabkhas or salt pans are inevitably low ground compared with the hinterland. As a consequence continental (fresh) waters often emerge as springs at the margins of such salt flats and dragonflies may be seen in such situations. It is always possible, of course, that a saline lake became temporarily freshwater. Drastic salinity fluctuations are shown by a chert with charophytes, freshwater molluscs and evaporites in the basal Purbeck Formation of Portesham (West, 1961; Barker, et al., 1975).

Modern salt-encrusted sabkas often contain both insects stuck in the slightly damp, adhesive salt and footprints preserved in the dried crust. The proximity of Purbeck fossil insects to salt crusts is suggested by their occurrence in some abundance at Coombefield in the calcareous shale just beneath the Hard Slat, a bed of the Hard Cockle Member that contains at its base dinosaur footprints in a salt-crust.

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QUATERNARY DEPOSITS:

- Bones in the Portland Fissures

Vertebrate remains have been found in the fissures in the Portland Stone, mainly in the northern part of the Island. Neale (1852), for example referred to discoveries when a cutting was made in the northeastern part of Portland for the conveyance of stone for the construction of the Portland Harbour breakwaters. The cutting was to about 30 feet (10 m.) and encountered a fissure with blocks of a black stone and a molar tooth of a horse.

ACKNOWLEDGEMENTS

I am very grateful to the Richard Edmonds, Paul Ensom, Justin Delair, Mark Godden (Quarry Manager of Albion Stone), and the late Michael House for help and discussion with regard to Portland topics including Purbeck facies and dinosaur footprints. Several former students, including Jennie Neve, Simon Wooster, Caroline Clasby, Jenni Will and Josephine Walker have provided assistance and valuable help in the field and laboratory with regard to Portland geology. This is very much appreciated. The painting of Iguanodons is by Anthea Dunkley and I am very grateful to her for permission to use it. Louse Tanner's work on Corallian carbonate has been of much value and I am grateful for her help. I very much appreciate the kindness of Ken Van Dellen of Michigan in providing superb aerial photographs of the Isle of Portland and the Chesil Beach. I very much appreciate the help of John Needham and Izzy Needham for showing me the silicified tree remains, resembling those of the Isle of Portland, but which they have discovered in Wiltshire.

I particularly thank John and Jane for much assistance when exploring the rocks near Cheyne Weare.

BIBLIOGRAPHY, References and Internet Links

Go to Bibliography and References relating to the geology of the Isle of Portland.

Go to Bibliography and References relating to the Chesil Beach

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Ian West has been awarded the R.H. Worth Prize for 2008 of the Geological Society of London for the application to amateur geology of the website, the Geology of the Wessex Coast, of which this webpage is a part.