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Monday, 18 April 2016

Evidence for a pre-Devensian glaciation of the Isles of Scilly

Faceted erratic cobbles (probably glacially transported) found in the granite breccia / lower head on the SE side of St Agnes, well outside the supposed Devensian ice limit.

 Pink sandstone erratic boulder on the coast to the east of Watermill Cove, St Marys Island, more than 4 km south of the supposed Devensian ice limit.

There is great confusion about older glaciations in the Isles of Scilly.  According to Scourse (1991) there was a more extensive  pre-Devensian glaciation that affected the islands.  Quote from his Abstract: “........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”.   However, Hiemstra et al (2006) claim that “the evidence suggests that an Irish Sea Glacier has only reached so far south on one occasion.”  By this they mean that there was no pre-Devensian ice incursion across the islands. James Scourse was one of the authors of this later paper.

This latter claim of "no earlier glaciation" is not reliable and is not supported by the evidence.  There are erratics in all of the raised beaches of the islands, some of them very far travelled -- and faceted, sub-angular and sub-rounded pebbles and cobbles are found in coastal exposures well beyond the Devensian ice limit.

For example, there are small faceted erratics in the “lower head” and granite grus on both sides of Beady Pool on St Agnes Island, and near the southern tip of the island small faceted pebbles of red sandstone, grey shale and bluish rhyolite have been found in the granite breccia, 2m beneath its upper surface.  These pebbles could not have had anything to do with the Devensian glacial incursion.

By the same token, erratics found in raised beach exposures must have been present on the islands prior to the last interglacial episode.  There are many erratics in the raised beaches around the head of Porth Killier bay, near the northern tip of St Agnes Isle.  On the east side of Gugh Island a striated cobble of chert (?) has been found deeply embedded in rotten granite breccia or grus, in a position that could not have been related to the Devensian glaciation.   On the east coast of Bryher, on the shore of New Grimsby Harbour, an erratic cobble of red sandstone was found deeply embedded in gravel breccia or head. At Tregear’s Porth, east of Watermill Cove on St Mary’s island, there are abundant red and pink sandstone boulders which must have come from the north.

 Grooved and striated chert (?) pebble found on the east side of Gugh Island, embedded in granite breccia / lower head.  This may be a beach pebble picked up and redeposited by ice, and then incorporated into slope deposits.

There are many references to these “old” erratics in the older literature, and I have to agree that the evidence on the ground points to the presence of glacier ice predating the last interglacial and the Devensian. At least one glaciation was so extensive that ice covered all of the islands.  Evidence from elsewhere suggests that the Anglian Glaciation is the best candidate -- since glacial deposits of this age are already well known from other parts of SW Britain. 

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