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Saturday 13 January 2024

Glacial erratics on Samson, Isles of Scilly


I have speculated that Late Devensian (LGM) glacial ice may have affected the western flank of Samson, and I have previously reported that Dave Mawer has found glacial deposits there.  Dave has now very kindly sent some photos which we can place on the record.  The approx location on the west (seaward) side of South Hill is shown on the satellite image above.

On one of the photos Dave has marked the two most obvious erratics -- one of flint and the other of sandstone -- but if you look carefully at the photos there seem to be others as well, of various shapes and sizes.  They are embedded in "ram" in a sandy and silty matrix that shows signs of stratification.  This suggests that the ram here is an accumulated layer of colluvium -- windblown and washed slope material.  So I don't think this is an original glacial deposit dating from the LGM, much as I would like that to be the case!  Maybe the deposits here are indicative of an ancient glaciation, rearranged or redeposited during the Devensian.  Or if the glaciation was indeed from the LGM, leaving deposits from the time of maximum ice extent (around 26,000 years ago), were 10,000 years or so of ongoing periglacial conditions sufficient to rearrange any thin deposits laid down here on a sunny and moist west-facing hill slope?  A delicious dilemma.......

Dave also says he has seen erratics in the ram on the west side of North Hill on Samson. One of his photos (below) shows a gravelly sandy diamicton containing erratic pebbles of many different types, shapes and sizes.  This looks to me like a thin coherent till resting on a cemented layer with manganese oxide staining.  Late Devensian / LGM?  The deposit needs to be examined properly........


Coherent LGM till on the west side of North Hill, Samson?  Clasts of many lithologies, shapes and sizes.  Photo: Dave Mawer.  This is the best evidence I have seen of a Devensian glacial deposit on this particular island.

West side of South Hill, Samson.  Photo courtesy Dave Mawer.  The locations of the two most prominent erratic cobbles are shown.


Closeup of the rounded flint erratic


Closeup of the red sandstone erratic.  Note that there are also other rounded and sub-rounded pebbles in this deposit.








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