James Scourse on the Late Pleistocene of the Isles of Scilly

This is the key publication on the Late Pleistocene of the Scillies -- unfortunately it is behind a paywall.

Late Pleistocene Stratigraphy and Palaeobotany of the Isles of Scilly

J. D. Scourse
Phil Trans Roy Soc B, December 1991
Volume: 334 Issue: 1271
Published 30 December 1991.
DOI: 10.1098/rstb.1991.0125
    •
Abstract

A re-evaluation of the Pleistocene stratigraphy of the Isles of Scilly has enabled the formal definition of eight lithostratigraphic units of member status grouped into two formations. A chronology of events has been provided by radiocarbon determinations, optical and thermoluminescene (TL) dates. Intersite correlations have been strengthened by palynology, which has aided palaeoenvironmental reconstruction. The defined units have been incorporated into two lithostratigraphic models, one for the `northern' (glacial) Scillies and one for the `southern' (extra-glacial) Scillies. Raised beach sediments of the Watermill Sands and Gravel in the southern Scillies are overlain by the Porthloo Breccia, a unit of soliflucted material derived exclusively from the weathering of local granite. Organic sequences at Carn Morval, Watermill Cove, Porth Askin, Porth Seal and Bread and Cheese Cove occur within the Porthloo Breccia, and are interpreted as the infillings of ponds associated with active solifluction. Radiocarbon determinations from these organic sediments are critical because they pre-date units associated with a glacial event. The C14 age determinations indicate deposition of the organic material between 34500 and 21500 years BP and provide a maximum age for the glacial event and the first radiometric dates for the coastal `head' sediments of southwest England. The pollen assemblages from these organic sites all record open grassland vegetation, and represent the earliest vegetational record for the Scillies. High Pinus values are interpreted as evidence of climatic deterioration. In the southern Scillies, the Porthloo Breccia is overlain by the Old Man Sandloess, a coarse aeolian silt with subdominant fine sand, TL-dated to 18600 years (Wintle 1981) and optically dated to 20000 and 26000 years (two samples; Smith et al. 1990). This material occurs in a variety of facies related to different modes of reworking. In the northern Scillies, the Porthloo Breccia is overlain by three units that are all related to a single glacial event. The Scilly Till, a massive, poorly sorted, clay-rich pale brown diamicton containing abundant striated and faceted erratics of northern derivation, occurs at Bread and Cheese Cove, and Pernagie and White Island Bars. This sediment is of uncertain depositional facies, although the available data suggest that it may be a lodgement till. At Bread and Cheese Cove the Scilly Till occurs in association with a matrix-supported sandy gravel, the Tregarthen Gravel, which has an erratic assemblage consistent with the underlying Till. The distributional relation between the glacially derived sediments, marine bars and morphological varieties of granite tors suggest that some of the bars may be remnant moraines, and that the glacier was erosive in the northern Scillies. Aeolian loessic processes in association with the glacial advance resulted in the deposition of the Old Man Sandloess in the southern Scillies. The relative coarseness of  this material is interpreted as a function of its proximity to glacially derived source material. The mineralogy of the Scilly Till is sufficiently similar to the Old Man Sandloess to suggest a genetic link between the two units. Overlying the Scilly Till and Tregarthen Gravel in the northern Scillies is the Hell Bay Gravel, an extremely widespread matrix-supported gravel containing a similar assemblage of striated and faceted erratics to the underlying Till, but alongside a considerable proportion of locally derived granitic material. The matrix of the Hell Bay Gravel is identical to colluvially reworked facies of the Old Man Sandloess. This material represents an initial phase of solifluction, post-dating the glacial event in which the Scilly Till, Tregarthen Gravel and Old Man Sandloess were mixed and transported downslope. In situations where these sediments were stripped from the land surface, weathered granite once again became the dominant raw material for solifluction, this subsequent phase being represented by the Bread and Cheese Breccia in the northern Scillies and the upper Porthloo Breccia in the southern Scillies. The evidence therefore suggests that ice advanced at least as far as the northern Isles of Scilly during the Dimlington Stadial of the late Devensian Substage. This conflicts with previous interpretations which place the glacial deposits within the Wolstonian Stage. However, the late Devensian event was probably not the first glacial event to have influenced the Islands because erratics are widespread in some exposures of the Watermill Sands and Gravel; the age of this earlier event remains uncertain.

The glaciation of Bryher, Isles of Scilly

Bryher and Tresco, two of the islands of the Northern Scillies.  The blue line shows the ice edge as shown by Mitchell amd Orme, Scourse, Hiemstra and others.  The red dot shows the position of the Popplestones Moraine, and the green dot shows the position of the till exposed in cliff sections just outside the moraine, in Little Popplestones Bay. [Note that in the post of April 18th, the accuracy of this line is subjected to scrutiny.]

From first observations on Bryher, there is little doubt that the northern and north-western parts of the island have been affected by ice during the Devensian.  

On the east coast, south of Norrard and The Bar, there are many exposures of reddish gravelly solifluction materials in the low cliffs.  This material is generally less than 2m thick in exposures, and there are no clear stratigraphic breaks within it, although in some sections the base seems to consist of rotted granite more or less in situ with soliflucted materials above, grading upwards into more sandy (windlown?) materials and then a darker organic-rich horizon near the surface.  Some erratic pebbles are seen on the beaches and occasionally in the cliff sections -- red and pink sandstones, limestone, grey shales and mudstones, and some schists.  There are many different types of granite -- probably mostly local. I would support the conclusion of other authors that there are traces of ancient glacial deposits -- suggesting a past more extensive glaciation than that shown by the glacial deposits further north. 

Reddish gravels exposed on the east coast of Bryher, south of The Bar. There appears to be a greater proportion of windblown material in the upper part of the section, beneath the organic-rich soil horizon.

Reddish gravelly slope deposits exposed in the cliffs of Rushy Bay, at the southern end of Bryher.  There are no in situ or redeposited glacial deposits here.  The gravels are partly cemented by iron and manganese oxides, and blocks of this material have been falling from the cliff face.

Following the cliffline northwards (not very systematically, because of dodgy weather and companions with other priorities!), glacial deposits are encountered for the first time in Popplestones Bay, towards its northern end.  Suddenly the sedimentary stratigraphy becomes much more interesting, as shown in the photos below.

Google Earth image of Popplestone Bay, on the west side of Bryher.  Here there is a prominent moraine ridge (shown by the solid red line) and outside it a thin layer of till is exposed in  the cliffs at approx the position of the red dots.  A speculative maximum ice limit for the Devensian is shown by the dashed red line.  It is assumed that a small ice lobe more or less filled the bay. (Addition 18th April:  glacial deposits have now been discovered on the south shore of the bay.)

Popplestones Bay cliff section, outside the Devensian glacial limit.  At the base, stratified gravels grading up into finer unstratified sandy deposits and then an organic rich soil horizon.  The modern beach is at the base of the photo.

 Little Popplestones, inside the Devensian ice limit.  At the base, coarse solifluction materials with mostly local bedrock inclusions.  Above a highly irregular junction, gravelly till packed with clasts of many shapes, sizes and lithologies.  Above that, a finer sandy and silty deposit with scattered stones derived from local bedrock and from the till.  Above that, a dark organic-rich soil horizon. 

A few metres to the north and west of these exposures there is a prominent moraine ridge flanked on its northern edge by the modern storm beach.  The surface of the moraine is littered with boulders and stones of many shapes and sizes, including many that are heavily abraded and faceted.

The Little Popplestones moraine, on the form of an elongated mound with surface till exposures and a litter of abraded and faceted boulders.

 In the literature, many different names are given to the deposits shown above.  The reddish gravels which make up the bulk of the cliff deposits outside the ice limit are called the "Porthloo Member" by Scourse and others.  The till is referred to as the "Scilly Member" (with a stratotype at Bread and Cheese Cove on St Martin's Isle).  The deposits above the till are referred to by a variety of names which may or may not be appropriate for Bryher -- judgment is reserved......

Anyway, thus far I'm quite happy with the conclusions of other authors, except to say that the Devensian ice extended somewhat further than is suggested on earlier maps.  I think this is the first record of till and a constructional moraine at Popplestones Bay, so that's my small initial contribution to the Ice Age history of the Isles of Scilly........

Glaciation on the Isles of Scilly

 Striated stones from glacial deposits on the north coast of St Martin's (credit:  Peter Knight)

Here we are on the Isles of Scilly, and already I have found glacial deposits outside the ice margin as shown on the maps of previous field workers.  More of that anon.  In the meantime, this is a nice summary from Peter Knight.

From Peter Knight's web site:
http://www.petergknight.com/scilly/glaciers.html

If you know Scilly at all, you will be familiar with the gravelly orange cliffs of "ram" or "head" that overlie the bedrock all around the islands' coasts. This is NOT glacial sediment, so don't get excited. Actually it is still quite exciting, as it is what we call a "periglacial" deposit, created by mass movements (soil creep and landslides) when the ground was going through periods of thawing and freezing during the ice age when glaciers were nearby but not actually covering the islands. The glaciers only seem to have covered a tiny little bit of the north of Scilly, and that is where you can find the glacial sediments: the "till" that contains the erratics. The glacial sediment generally has more clay and silt in it compared with the dominantly sandy gravel of the ram or head around the rest of the islands, but both the till and the ram have lots of stones and boulders in, so it is not always easy at all to tell the difference! In section of cliff on St Martins, there is a layer of till sandwiched between layers of ram above and below it. What geologists call the "type site" for the Scilly till, in other words the location where they had a good look at it and were able to describe and define it properly, is Bread & Cheese Cove on the north east side of St Martins. In fact, this location features in a guide to the UK's most important Geological Sites relating to the history of the last 2 million years or so that was published in 2015 (see reading list below). Over the years a number of professional (and amateur) glacial geologists have explored the northern coastlines of Scilly looking for glacial deposits, and it seems that you might strike lucky anywhere along that thin strip of coastline across the northern perimeter of Scilly from the eastern end of St Martins around to the northern coasts of Tresco and possibly even Bryher. It has been suggested that the White Island bar might have been pushed up as a ridge of rocks by the bulldozer effect of the front of the glacier, and I think (although I have not seen this published anywhere) that the ridge, a few metres high and a few hundred metres long, that runs parallel to the coast around the north west corner of St Martins overlooking White Island looks very much like a glacial moraine (a ridge of debris deposited or pushed up by the ice). There are is also a layer of gravels on Tresco (called the Tregarthen gravels!) that are supposed to have been deposited in streams running off the glacier, and there is a line of boulders at Shipman Head on Bryher that is supposed to be a glacial moraine: a ridge of debris pushup up by the edge of the ice. Some people have argued that the top of Round Island shows signs of glacial erosion, indicating that the glacier actually ran over the top of it.


Key references on the glaciation of the Isles of Scilly:

• Scourse, J.D. (1998) The Quaternary History of the Isles of Scilly (Chapter 8 in "Quaternary of South-West England", edited by S. Campbell et al.) Joint Nature Conservation Committee; Springer Science.
 • McCarroll, D. and others (2010) Exposure-age constraints on the extent, timing and rate of retreat of the last Irish Sea ice stream. Quaternary Science Reviews, Volume 29, Pages 1844-1852.
 • Hiemstra, J.F. and others (2006) New evidence for a grounded Irish Sea glaciation of the Isles of Scilly, UK. Quaternary Science Reviews, Volume 25, Pages 299-309.

Isles of Scilly on the agenda

 Illustration from Hiemstra et al, 2006

Tomorrow I'm off for a short holiday on the Isles of Scilly with my wife and four old friends.  Should be fun, if it's not too cold and wet.......

I have mentioned the islands many times on this blog, but have never been there before -- and I hope I can escape from the madding crowds now and then in order to do a bit of geomorfin'........

There are some rather interesting questions still to be answered with respect to this archipelago.

1.  Was the glacier ice that reached the islands in the Late Devensian (the dating seems to be pretty secure) really in the form of a "surge lobe" which pushes southwards from the St George's Channell to the Scillies and beyond?  As I have said many times before, this defies some of the laws of glacier physics, and I have real problems in accepting that a lobe like this could have had the form shown in the top left diagram. There must have been a calving bay with a concave shape at its outer end, and it must have spread laterally rather than flowing as a narrow lobe.  (Ice only flows in the manner shown when it is constrained within a clearly defined channel.  Such a channel does not exist in the Celtic Sea, and there is no evidence that it ever did.)

2.  To what extent are the curving depositional features on the northern islands related to actual moraines that once marked ice edge positions?  The evidence presented in various papers looks pretty convincing....

3.  Does the limit of the Hell Bay Gravel actually delimit the maximum ice extent during the Devensian?  Ice limits are often more difficult to define than one might imagine!

This map is from the Isles of Scilly chapter by James Scourse, published in the GCR review for SW England (1998).  Many other authors have suggested similar limits, and it will be good to see some of the evidence on the ground.

4.  In most of the recent literature on the glaciation of the Isles of Scilly it is claimed that the Devensian Irish Sea Glacier had a greater extent (at least in the Celtic Sea arena) than any of the earlier glaciations of the current ice age.  Scourse claims this, as do others with whom he has worked -- and yet there appear to be "widespread" erratics in some exposures of the "Watermill Sands and Gravels" which  underlie and pre-date any of the Devensian periglacial and glacial deposits on the islands.  Was there an older and more extensive glaciation, or was there not?  It will be interesting to check out the evidence on the ground. (Watermill Cove, the type locality for these early deposits, is on the isle of St Mary's, where we are staying -- so I shall certainly check it out.  It's well to the south of the Devensian glacial limit as currently defined.)

5.  How many raised beaches are there?  Some writers say one, and others say two.......

6.  What are the similarities between the Isles of Scilly and Pembrokeshire?  The Devensian stratigraphy does appear, from the descriptions, to be closely correlated.

The Pwllderi cliffs

When I was over at Pwllderi, on the west side of the Pencaer Peninsula, the other day, I was suddenly struck by the extraordinary degree of ice moulding on the west-facing cliffs.  Features like these would not be out of place on the coasts of Greenland or Antarctica, and they indicate just how powerful glacier ice can be when it is moving directly onto an old coastline (here from the NW) and is being forced to move upslope.  So there must have been powerful abrasion, a great deal of entrainment of loose material,  and a lot of shearing within the ice mass under compression.

Much of the coastline between Abermawr and St David's Head looks like this, and hillocks on the edge of the coastal plateau have also been smoothed off and rounded.  I must do some more work on this, when I can find the time...... and one interesting question (one of many) is this:  to what extent are these Devensian features, and to what extent are they inherited from earlier glacial episodes?

Rainbow ice

This is rather freaky -- saw it on a web site with no explanation of the phenomenon, or location.  I suspect it is somewhere in Antarctica.  Quite wonderful!!

Martha Morgan Country now launched

 In the footsteps of Mistress Martha.  Model: Rhiannon James.  Photographer: Steve Mallett

Excuse my absence from the blog in recent days -- it's been all hands on deck for the launch of my new "literary tourism" project centering on the books of the Angel Mountain Saga.  That was yesterday, at the Boat Club in Newport, and it was a great success -- a packed room and support from all over the place for our attempts to bring more visitors to the area, focussing on the "heritage" features of the landscape.  The stars of the show were model Rhiannon James and photographer Steve Mallett, who are jointly responsible for a fantastic portfolio of photos which can now be seen here:

http://www.marthamorgan.co.uk/photo-gallery.html

Actually, Rhiannon is not a model at all, but a very attractive and very smart young lady who is studying law in Cardiff University at the moment.  She is currently Miss Pembrokeshire, and she's also a very close match for the heroine Mistress Martha Morgan in the imaginations of many readers.......

Here is one of the "heritage" pages from the new web site:

Here is some of the press coverage:

http://www.countyecho.co.uk/article.cfm?id=103884&headline=Author%27s%20eight-novel%20saga%20at%20heart%20of%20new%20tourism%20project&sectionIs=news&searchyear=2016