The Purbeck Fossil Part 2: the Forest Trees, east of Lulworth Cove, and Isle of Portland

West, Ian M. 2018. The Fossil Forest - Part 2 - Trees, Lulworth Cove and Isle of Portland; Geology of the Wessex Coast, field guide. Internet site: www.southampton.ac.uk/~imw/Fossil-Forest-Purbeck-Trees.htm. UK. Version: 14th June 2018.

The Purbeck Fossil Forest - Part 2 - Trees - east of Lulworth Cove, and Isle of Portland, Dorset. Geology of the Wessex Coast

This is:
THE FOSSIL FOREST: PART 2: THE PURBECK FOSSIL TREES.
For Purbeck Strata of the Ledge - go to:
THE FOSSIL FOREST: PART 1: GEOLOGY OF THE LEDGE.

by: Ian West
Romsey, Hampshire,
and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University
Website hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , Southampton.
Website archived at the British Library

Click here for the full LIST OF WEBPAGES

(You can download this educational site to SurfOffline or similar software to keep an offline copy, but note that updating of the live version takes place periodically.)

INTRODUCTION TO THE FOSSIL FOREST

This is

The Purbeck Fossil Forest, Part 2.

For matters of:
Location, Access, Safety and general geology at the Fossil Forest Ledge near Lulworth Cove please go to the related webpage:

The Purbeck Fossil Forest - Part 1:

Fossil Forest Ledge, Description of the Exposure with Thrombolites, Evaporites etc.

Note particularly there is some risk of falling into the sea if you stand near the edge and pay no attention to the hazard of the cliff. It is easy, almost normal, that when engaged in geology one is busy and may take no interest in the presence of a cliff edge. It is thus not uncommon to walk backwards when photographing and to ignore the edge. The Fossil Forest seems completely safe but it is not actually so secure a place. There have been fatalities around here, although not for this reason.

See also:

LULWORTH COVE WEBPAGES:

Lulworth Cove Geology - Introduction
Lulworth Cove Purbeck Group, West Side of Cove
Lulworth Cove Purbeck Group, East Side of Cove
Lulworth Cove - Stair Hole
Lulworth Cove - Fossil Forest - Introduction
Lulworth Cove - Fossil Forest, Part 2 - Purbeck Trees (this webpage)
Lulworth Cove - Dungy Head
Lulworth Cove - Durdle Door to Bats Head
Lulworth Cove - Mupe Bay and Bacon Hole
Lulworth Cove Select Bibliography

INTRODUCTION - GENERAL

The Fossil Forest is a limestone ledge with a late Jurassic soil and remains of rooted trees. It provides information on an ancient environment in which late Jurassic dinosaurs lived. It is in the cliff east of Lulworth Cove in Dorset, UK. The remains of fossil trees, about 140 million years old, can be seen, most are empty moulds, but there is some preservation as silicified wood. There are also stromatolites or, more strictly, thrombolites which encrust the trees and interesting evaporite and tectonic features.

See also the alternative location for fossil trees:

Purbeck Fossil Trees on the Isle of Portland.

For maps, safety information, general photographs, sedimentology, thrombolites, evaporites, Broken Beds etc. please go to the associated webpage:

Fossil Forest, Part 1 - Introduction and Sedimentology.

THE FOSSIL TREES OF THE PURBECK GROUP THE PURBECK FOSSIL TREES

Purbeck Fossil Trees - the Remains

(Note: regarding nomenclature - see some brief text further below regarding use of Pararaucaria as opposed to Protocupressinoxylon; the latter and more familiar name is still used in much of the text below. In any case, more than one tree species seems to be present in Dorset, so avoid simplification.)

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.

The Purbeck trees have been very well described by Professor Jane Francis, and seem to have been an early type of cypress or juniper type. The wood was named as Protocupressinoxylon purbeckensis. However, as discussed further below that the name "Protocupressinoxylon" is illegitimate, according to Philippe and Bamford (2008). These trees were apparently common in Jurassic times in the British area. They appear not have been confined to very dry, near-saline environments, but were able to tolerate a climate in which evaporites were deposited, at least in summer. The dry forests of Western Juniper trees in parts of the western USA have some similarity. Modern Mediterranean environments, such as those of Cyprus, are even better because the climate and latitude are more similar to the Purbeck palaeoclimate and palaeolatitude. In addition, they grow on limestone soils, with caliche and calcrete, close to seasonal salt lakes (e.g. Akrotiri salt lake). Unfortunately, most modern Mediterranean areas have been badly affected by human deforestation, and the tree cover is now very patchy and rarely entirely natural.

Southern Cyprus has lakes which are only of moderate or low salinity in winter but which precipite evaporites in late summer. These are quite good analogues for the seasonal Purbeck palaeoenvironment. Of course, the Purbeck lakes and lagoons were much larger and in a very subdued topography. This scale and topography is not matched in Cyprus. We know that the late Kimmerian movements were taking place in the Dorset area. These provide an explanation for the rapid flooding, which is suggested by the fact that the trees did have time to rot. Trees of Cypress type have resistant timber, but the hypersaline brine, indicated by evaporite remains above the Fossil Forest, probably had a preserving effect. It is probable that the salt lake quite suddenly extended over the former forest, killing the trees and pickling them prior to silicification.

In several places in Dorset are the remains of a late Jurassic forest quickly killed by a flood of very saline water which swirled around the bases of the trees. The coniferous trees near the base of the Purbeck Group were in many cases replaced by silica. They are visible at various quarries on the Isle of Portland and at certain localities (not specified so as to preserve the trees) in the Weymouth area. The Fossil Forest, near Lulworth Cove, is excellent because here there is an easily accessible place showing numerous moulds of fossil trees surrounded by thrombolitic limestone. Partly because the silicification was not so widespread here and partly because of the unfortunate problem of collectors removing fossil wood there are now the partial remains of only one silicified trunk. This, known as the "Last Tree" is shown here as in 1981 when there was more silificied wood left. However, good moulds of trees are numerous. The Fossil Forest section is an important and very special site for showing the nature of the soil and associated caliche, the distribution of the trees, the domal thrombolites developed above them and the evaporitic deposits of the salt lagoon in which they were submerged. The Fish remains are found above the forest and are referred to below.

This is an example of part of a silicified Purbeck tree trunk. This particular specimen was studied by Professor Jane Francis (1984) and provided important evidence from the tree rings for seasonal Mediterranean type climatic conditions. It was found on the cliff top ledge (" Fisherman's Ledge" of the rock climbers - Crewe, 1977) between Anvil Point and Blackers Hole, near Swanage. It was not in situ and since silicified tree remains have subsequently been found in the Middle Purbeck , just above the Cinder Bed, in Durlston Bay it is not certain that this specimen is from the basal Purbeck.

Here are the remains of a horizontal tree in the Great Dirt Bed of Portland. The timber has been replaced by chalcedony. This fossil wood is near the eastern end of the Withies Croft Wall in a quarry on the Isle of Portland. The silicified timber is orientated in the direction 301 degrees (true bearing). There was probably originally much silificied wood, like this, present at the Fossil Forest Ledge, but unfortunately, it has all been removed by collectors. Of course there is still much more underground and some of this will be exposed by erosion in the future.

Tree remains from the Isle of Portland have been found in rock armour at Highcliffe. The uppermost Portland Limestone, the Roach and some of the basal Purbeck limestone above have been used as rock armour in the region. There is much in the Highcliffe area. Thrombolitic material is common. Shown above is a hollow mould of a basal Purbeck coniferous tree as seen at the outflow of Chewton Bunny (a small stream) at Highcliffe.

Left: A shoot from a Purbeck tree. The specimen, from the Day Collection at Southampton University, is from a sandy limestone of the Purbeck Group of Lulworth Cove. It contains bivalve shells, possibly Neomiodon and a some small vertebrate tooth. Dispersed through the limestone is quartz sand, and this is visible in the photograph. The horizon is not known but I would expect it to have been Middle or Upper Purbeck, rather than lower (which has finer-grained limestones usually lacking in teeth). Francis (1983) has described a rather similar shoot from the basal Purbeck of the Isle of Portland, and some similar features can be seen in this. ' Branches arise alternately at every node on the main stem and minor branchlets, but appear to be arranged in opposition on the ultimate shoots. Alternate branchlets are either straight or slightly curved. The main laterals subtend an angle of about 45 degrees to the main stem and subsequent branches subtend an angle of about 55 degrees. The whole shoot is now strongly compressed and bears small leaves in an opposite and decussate arrangement strongly reminiscent of modern Cupressaceae, such as for example, Thuja plicata D. Don.'

Right: Comparison of the foliage of the Purbeck trees with that of common Cypress trees. The foliage was identified by Professor Jane Francis (1983) as of Cupressinocladus valdensis, a Cupressus-type foliage already known from the Lower Cretaceous Wealden. The two insets are modified from Francis (1983) and are fossil specimens at different scales. Colour has been changed for comparison with the modern specimens. There was a general similarity to the modern foliage of cypress or juniper trees.

More comparison with modern trees can be made. As shown above, the relatively dry forests of Western Juniper of Oregon and California have the similarity of not only, perhaps, a distant taxonomic relationship but also have the ability to survive in proximity to semi-desert areas with salt lakes. Of course, these interior environments are mostly of too high altitude and rather too high in latitude. However, they are in very seasonal environments, like the Purbeck forest palaeoenvironments.

PURBECK CONIFEROUS TREES - TAXONOMY

With regard to Purbeck trees, it is known that there are problems of illegitimacy of nomenclature. One comment draws attention to this. The second argues for interim retention. It is intended to follow the second instruction here until or unless different guidance is given.

Some simple taxonomic advice regarding Purbeck trees from 2008 and 2010 is given below, but particularly see the second section. Please note that this is just informal guidance, based on recent literature, but is not necessarily up-to-date when read. References are given to source. If there is new and official taxonomic guidance given which differs from this or updates the situation, then that should be followed, rather than this.

THE PURBECK CHEIROLEPIDACEOUS FOSSIL TREES - TAXONOMY

(1) Illegitimacy of Protocupressinoxylon (2008)

There has been taxonomic literature that concerned the Purbeck trees ( Philippe and Bamford (2008). They stated that the name Protocupressinoxylon is illegitimate and a comment by those authors is given below. However, see also the retention guidance which follows after this and that is recommended for the present, pending more specific taxonomic intructions.

"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."

[See also a further note below relating to Steart (2014) suggesting that the Cheirolepidiaceous Purbeck trees produced Pararaucaria cones, at least in the Vale of Wardour.]

THE PURBECK CHEIROLEPIDACEOUS FOSSIL TREES - TAXONOMY continued

(2) Guidance for Interim Retention of Protocupressinoxylon purbeckensis, (2010)

With regard to taxonomy of Protocupressinoxylon see the following, taken from p.208 of Philippe et al. (2010). This is a useful and clear explanation of the situation in 2010. Should the present writer becomes aware of a change from this, then updated guidance will be given. Meanwhile the following will be applied here.

"There is a nomenclatural problem with Protocupressinoxylon Eckhold. This morphogenus is a junior nomenclatural synonym of Protobrachyoxlon Holden (Philippe 1993) (but not of Paracupressinoxylon Holden; Philippe and Bamford 2008). Thus, and despite that Protocupressinoxylon was frequently used, it is illegitimate. Conservation has never been proposed for this genus. Several problems have prevented the proposal for such a conservation. First, all material for Protobrachyoxylon eboracense Holden, the type species of the genus Protobrachyoxylon has been lost (Philippe 2002). Second, from the elements given in P. eboracense protologue, genus Protobrachoxylon is very probably a taxonomic synonym of Brachyoxylon Hollick and Jeffrey, and thus genera Protobrachyoxylon and Brachyoxylon are very probably taxonomical synonyms. Thirdly, Protocupressinoxylon has been used in very different ways, depending on whether authors paid more attention to one or another of the Eckhold's syntypes or Eckhold's diagnosis which unfortunately does not describe oculipore arrangement in cross fields. Fourth, the genus Protopodocarpoxylon was often used for material with araucaroid cross-fields and mixed type of radial pitting (just as in Brachyoxylon Hollick and Jeffrey), in line with the conditions observed by the type material for Cedroxylon blevillense Lignier, one of the Protopodocarpoxylon Eckhold syntypes (Lauverjat and Pons 1978). A nomenclatural and taxonomic reappraisal of Mesozoic wood genera is currently in progress by one of us [i.e. one of: Philippe, Billon Bruyat, Garcia-Ramos, Bocat, Gomez or Pinuela]. Meanwhile we [i.e. Philippe et al. (2010)] will use herein the name Protocupressinoxylon purbeckensis Francis, which is well-known and based on well-described material."

[END OF PURBECK TREES - TAXONOMIC SECTION

THE PURBECK PINE CONES

Purbeck Fossil Pine Cones -
Portesham and Vale of Wardour,
Araucarites and Pararaucaria cones

Portesham Cones

The Portesham Charophyte Chert Bed is a remarkably rich fossil horizon in the basal Purbeck strata that I found in 1961. Later that year I dug out more material with the help of Professor Sylvester Bradley. We used pickaxes. The bed provided abundant palaeontological material. This was was studied by Barker et al. (1975) (and I turned down the option to be a co-author). More recently another cone has been found more recently by John Needham in the Vale of Wardour. Some very thorough technical details of the new cone have been published. The reference to this recent work, follows this small section on the Portesham cone. See: Steart et al. (2014).

There follows an extract from Barker et al. (1975) who studied the cone and other good fossils in the bed which I very briefly reported in Nature (West, 1961 - Lower Purbeck Beds of Swindon facies in Dorset. Nature, London.)

"CONIFERALES, ARAUCARIACEAE
Araucarites sizerae sp. nov.
Plate 58, figs 21, 22, text-figs, 4, 7, 8.
Diagnosis: Cone presumed to be spherical, about 2 cm in diameter. Axis of uniform width, 3mm in diameter for most of the length, tapering at the upper end. Cone scales in a spiral, parastichies, possibly 3-5. Stalks of cone scales round in section, those at the lower end bent slightly downwards. Outer surface of scales rhomboidal 3-4 mm from corner to corner, with a low, rounded median boss 1-2 mm. wide. Seeds embossed singly in cone scale with pointed end towards the axis; seed about 4mm. long (including cone scale boss), 3 mm. at wider end.
Holotype. British Museum V. 44931.
Description: [not given here - see the paper]

The cone is similar to that of a modern Araucaria, though there are no long pointed extensions of scales projecting outside the cone. These may have been rubbed off. No ligule was seen but this is easily missed in such a fossil. The possibility was considered that the isolated seed called Carpolithes westi might be the basal seed-bearing part of the cone scale of the present cone. This still remains a possibility, but it was decided not to make them one species, because they do differ. It may be that the cone fragments are immature while the isolated C. westi are mature but there is no evidence of this. The smallest of the isolated seeds included in C. westi is as large as the typical A. sizerae seed and most are considerably larger (6-7mm). Their shape is also different, they are less flattened.... [continues].

Vale of Wardour Cone

Like the Portesham fossils, a silified cone of a Purbeck tree has been found by John Needham in the basal Purbecks of the Vale of Wardour, and technical details have been described (below) in 2014. The bed from which it has come might, perhaps be the equivalent of the Great Dirt Bed of Dorset, but, unlike the case of Portesham, it cannot be directly correlated and thus there is no proof of this. Because of water-level fluctuations in the same broad basin, it is quite possible that the palaeosol at the Vale of Wardour really is at the same horizon, but specific evidence for this is not, at present, available.

The cone has has been subject to laboratory study described in a seven-authored, technical paper by Steart et al. (2014). The abstract is given in the Purbeck Bibliography. This paper describes how the ovuliferous scale of the (slightly worn) cone have been subjected to X-ray synchrotron microtomography (SRXMT), performed at the Diamond Light Source. A three-lobed ovuliferous scale and ovules enclosed within pocket-forming tissue, have demonstrated an affinity with the Cheirolepidiaceae. Details of vascular sclerenchyma bundles, integument structure, and the number and attachment of the ovules indicate greatest similarity to Pararaucaria patagonica and Pararaucaria carrii. This is the first evidence for ovulate cheirolepidiaceous cones in Europe. It accords with the Portesham implications that the the Dorset, Cheirolepidiaceous, fossil forest trees had cones of rather Araucarian type. It is more specific in naming the cone as belonging to the genus Pararaucaria. It claims to be providing the first evidence for ovulate cheirolepidiaceous cones in Europe. It is not clear how the earlier described Portesham cones relate to this, in view of the fact that Araucarites sizerae and Brachyphyllum were described from the Purbecks long ago.

PURBECK FOSSIL TREES continued:

Tempskya (Tree-Fern) Type of Silicified Tree Remains.

[Note that the fossil tree dicussed here has now been removed from the quarry for safe preservation]

Trees of Tempskya or Tree Fern type are not normally seen in the basal Purbeck fossil forest horizons. It is not known whether this really is a large tree fern, but it seems likely. Palaeobotanical study is needed.

Sicified wood from Purbeck Tempskya tree remains is found in the basal Wealden at Stair Hole, Lulworth Cove. This suggests two points:

1. Tree ferns were common in the Purbeck forests, although probably not particularly so on the low, carbonate rich evironments at the margin of the Purbeck lagoon.

2. The Purbeck fossil trees were silicified very early, i.e. before early Wealden times.

The topic will be discussed further, later.

THE PURBECK FOSSIL TREES

Other Purbeck Tree Remains - Durlston Formation (Cretaceous)

In addition to the Pararaucaria-type trees, there were also some rather different conifers present in the Purbeck Group. Remains of these have been found in the Durlston Formation ("Upper Purbeck") of Durlston Bay. Although not necessarily present in the fossil forest of the basal Purbecks at Lulworth and Portland, but they were undoubtedly somewhere in the region, perhaps where the climate was wetter. Information on Araucaria - type trees follows:

Plant remains found by Mr A.J. Holmes on 6th December, 1998 in a rock fall under the Coastguard Station at Peveril Point, Durlston Bay, Swanage. Tony Holmes reports that the bed appears to be DB 221 in the Unio Member. Further comments are given in the Durlston Bay, Unio Member section .

Left: The foliage appears to be of Araucaria type, that is with similarities to the foliage of the "Monkey Puzzle Tree " or Chile Pine. Above it is piece of foliage of Araucaria excelsa , the Norfolk Island Pine. Below is modern Cupressus foliage, to which previous finds of Purbeck fossil foliage have closer resemblance (see illustrations of Cupressinocladus valdensis in Francis, 1983). This is in natural colours.

Right: An image of the same foliage with higher resolution, for detail.

The foliage and sediment shown in false colours, to give an indication of the original appearance of the small piece of branch embedded in the shelly lake mud.

Trees of a type (Araucaria) that might have been the source of the foliage shown above, are illustrated here at the margin of the Purbeck lagoon.

PURBECK FOSSIL TREES continued:

Dungeness Quarry Tree, Portland

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%

**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 thrombolitic 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.

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 FOSSIL TREES continued:

Purbeck Cycadophytes

Silicified tree remains, or petrified wood, are well-known to occur at the two horizons in the basal Purbeck Group in Dorset. These are just above the Lower Dirt Bed and just above the Great Dirt Bed. Both of these horizons are probably within the uppermost strata of the Jurassic System. Silicified tree remains also occur higher in the Purbeck Group in the base of the Lias Rag (DB 116) of the Intermarine Member in Durlston Bay, Swanage. This is in Cretaceous strata - Berriasian. A piece of silicified trunk, 17cm width by 7.5 depth was found lying parallel to bedding in the Lias Rag by a student making a field study with me in 1994. This was subsequently cut by a diamond saw.

A sawn and polished cross section shows the heartwood and the sapwood of this conifer trunk. An unusual feature for the Purbeck Group is the preservation of some bark on the outside. It is not continuous but there are patches of it remaining. Notice the wider spacing of the tree rings in the heartwood compared to the sapwood. Notice also the medullary rays. The trunk has been compacted and mineralised tension gashes are present. For more information on Purbeck tree rings, with regard to basal Purbeck trees, see Francis (1984) - The Seasonal Environments of the Purbeck (Upper Jurassic) Fossil Forests.

Silicified tree remains have also been found in the same bed, the Lias Rag, by Mr. Trev. Haysom in his Purbeck stone quarry at Acton near Langton Matravers. As discussed below, Paul Ensom (2010) has commented on silicified tree remains in the Purbeck, well above the Caps at the base.

A very good quality, but ex situ tree trunk with good tree rings was found by the author about half a century ago at Connor Cove (Fisherman's Ledge), east of Blackers Hole. This was investigated in terms of tree rings by Professor Jane Francis, who was at that time a research student partly under my supervision. I assumed that the tree had come from the basal Purbeck strata. This was reasonable since at the time silicified tree material from higher levels was not known. The piece of tree trunk is slightly different from the fossil tree material that is common on the Isle of Portland (less cracked, blacker and with better tree rings).

Further study has shown that no well-defined dirt bed with trees seems to be present in that particular area, which is in Purbeck basinal evaporite facies (although the cliff tops are difficult to access or study in detail!). Some thrombolites are present in the basal Purbecks of the cliffs and these might contain tree holes (like those on Portland) but clear and obvious examples have not been seen.

Thus, further consideration suggests that it is now more likely that it has come from from the Middle Purbeck debris that has slumped down the hillside in periglacial conditions. Cherty Freshwater Limestone debris occurred adjacent to it.

The very close-spacing of the tree rings ( Francis, 1984) is more compatible with a basal (i.e. evaporitic) Purbeck environment. Some uncertainty remains, though, but a Middle Purbeck origin is now much more probable. Silicification is common in the Cherty Freshwater Limestone, and the variation in the climate is shown by the present of siliceous pseudomorphs after halite in this bed at the Fossil Forest and other localities.

Note that silicified wood is also common in the basal Purbecks of the Vale of Wardour. Silicified wood pieces (and jet) occur at the top of No. 3 Gypsum Seam (bed) in the Mountfield Inlier (Howitt, 1964). This Sussex occurrence is at about the horizon of the Broken Beds in Dorset.

SILICIFIED TREES IN THE MIDDLE PURBECK GROUP (CRETACEOUS)

Silicified tree remains are now known to be common in the Middle Purbeck (i.e. Cretaceous - Berriasian) of the Dorset coast and they not confined to the basal Purbeck (Jurassic - Tithonian), as originally thought. Ensom (2010) has reported silicified wood and carbonaceous material in his beds 76-77 at the base of the Marly Freshwater Member at Bacon Hole.

Trees might be expected to be common lower down in the Soft Cap, because this locality is very close to the Fossil Forest. However, Bacon Hole shows rapid, small-scale, facies changes probably because of proximity to the Late Cimmerian, boundary fault of the English Channel Inversion. Fossil trees are rare or absent here in the Caps.

The Middle Purbeck occurrence is of interest and ties in with Middle Purbeck occurrences elsewhere. Silicified wood can occur in the Purbecks at various horizons, even if its abundance in the Caps is most well-known.

There is some more information in the publication by Ensom (2010). He discusses RIGS quarry sections and the Bacon Hole coast section of the Purbecks. He has provided a short section on "Silicified Wood" on page 129 and 132. He found silicified plant material, possibly reworked from sieve residues of the Cherty Freshwater Member (DB 102 or equivalent) at Sunnydown Farm. As noted above it is present at the base of the Marly Freshwater Member at Bacon Hole. Ensom found pieces of silicified wood about 0.5m above the Downs Vein at Suttle's quarry. He also found it a short distance above the Downs Vein in Durlston Bay, Belle Vue Quarry, and is in the same part of the succession at Lander's Quarry. It is now known to be common in the Lias Rag both at Durlston Bay and inland. The Lias Rag is in the Intermarine Member a short distance above the Cinder Bed and in the lowest part of the Durlston Formation of the Purbeck Group (Middle Purbecks in older terminology). This was probably deposited just before the well-known palaeoclimate change to wetter conditions and the resulting influx of kaolinite and sand. It was probably deposited still in semi-arid type Purbeck conditions.

In summary silicified tree remains occur in the Marly Freshwater Member, the Cherty Freshwater Member, both below the Cinder Bed. They also occur just above the Downs Vein and in the Lias Rag, units which lie a short distance above the Cinder Bed.

PURBECK FOSSIL TREES OF WILTSHIRE

For a very interesting account of the important discoveries of good Purbeck silicified trees and roots etc. in the Vale of Wardour, obtain a copy of the book by John Needham. Details are given below:

.
Needham , J. E. 2011. Forests of the Dinosaurs; Wiltshire's Jurassic Finale.

The Hobnob Press, PO Box 1838, East Knoyle, Salisbury, SP3 6FA. By John E. Needham. ISBN 978-1-906978-01-3. 221 pp. Printed by Lighting Source. Paperback, 12 pounds, 95 pence from Amazon (in Jan 2012).

ACKNOWLEDGEMENTS

I am very grateful to the Head of School and the Staff of the School of Ocean and Earth Science for supporting the running of this website. I thank iSolutions for runnng the website from their server. The Channel Coastal Observatory, and in particular Dr. Travis Mason has been very helpful with regard to aerial photographs. I much appreciate the helpful discussion with various people on field trips in the past back to the late 1950s. I think Dr. Geoff Townson for kindly providing photographs of the Shark Bay stromatolites. I very much appreciate discussion in the field with the specialist on microbial mounds, Professor Dan Bosence. I particularly appreciate the boost to my career that was given by the late Professor Bathurst in the 1960s, as a result of a sedimentological field trip that I was leading here. My wife and family have provided support and assistance in various ways. I particularly thank Alan Holiday for having realised the similarity between the Purbeck thrombolites and the thrombolites of Lake Thetis.

PURBECK FOSSIL FOREST WEBSITES, PART 1 AND PART 2

This is the end of:

THE FOSSIL FOREST: PART 2: THE PURBECK FOSSIL TREES.

To return to the geology of the Fossil Forest Ledge - go to:

THE FOSSIL FOREST: PART 1: GEOLOGY OF THE LEDGE.

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Copyright © 2018 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged. The contents of this website are copyright and not available for use in online or other courses or examinations run by commercial companies that operate for profit.

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Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.

at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.