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Sunday, 10 April 2016

Raised beach at Carn Morval, St Mary's Island

 Exposure of cemented raised beach below extreme HWM, some metres from the base of the drift cliff, near Taylor's Island on the west side of St Mary's.

The type locality for the "Watermill Member" of the Scillies sediment sequence is at Watermill Cove, on the east side of St Mary's Island.  This is a "basal raised beach gravel" assumed to date from the last interglacial.  It is overlain by the poorly-sorted breccia made of gravel, sand and angular granite clasts of many different sizes, seen best on the shore of Porthloo Bay where it is up to 4m thick.

I have not seen previous mentions of other exposures of the raised beach on the west side of St Mary's, between Porthloo and Carn Morval.  Some of these exposures of the raised beach are well below current HWM, where they are distinguished from modern beach sediments by the fact that they are solidly cemented.

Closer to Carn Morval these cemented raised beach materials can be seen at the base of the sediment cliff, full of well-rounded erratic and local pebbles. 

In some sections it is apparent that this beach has been affected by over-riding ice, since the gravels and pebble bands have been disrupted and mixed with faceted erratics in a silt and clay matrix.

Solidly cemented raised beach gravel, packed full of well-rounded erratic and local pebbles.  At the base of the drift cliff south of Carn Morval.  At the top of the photo we see a large detached granite slab and the base of the fine-grained slope sediments belonging to the Porthloo formation.

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