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Sunday, 10 April 2016

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........


chris johnson said...

Would the islands have been joined to the mainland with a land bridge at the time?

TonyH said...

Never been, but I've heard it's possible to walk between the islands of Bryher, Tresco and Samson at low tide in Spring.

Incidentally, Saint Samson seems to have got around: Cornwall, Scillies and Pembrokeshire, if indeed this Samson island gets its name from the Saint.

BRIAN JOHN said...

Chris -- yes, a distinct possibility that the archipelago was connected to the mainland at the time of the Devensian glaciation. There is the legend of the Land of Lyonesse stretching from Lands End to Scilly. And Tony -- yes, we walked from Bryher to Tresco last weekend when the tide was low. We never got to Samson, but that can also be walked to at times of extreme low tides. It's amazing how all of the islands greatly expand in size as the tide drops.....