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Wednesday, 1 December 2010

Glacial deposits on the Lizard?

I have come across a couple of references in which it is suggested that the strange gravels at Crousa Downs, on the Lizard, are glacial deposits dating from some ancient (Anglian or earlier?) glacial episode.  The conventional explanation is that they are Tertiary fluvial gravels:

"There are superficial deposits lying on the Lizard rocks at Crousa Downs, Lowland Point, and as an apron of clay-bound gravels around the coast. Those at Crousa Down represent a Tertiary Age deposit of sands and gravels of probable fluvial origin, which have been derived from the north. These gravels contain traces of cassiterite and gold and would be classified as placers. At Lowland Point is a Pleistocene Age deposit of loess of wind blown silty sediments (this occurs elsewhere on the Lizard) and clay-bound gravels occurring as aprons around the coast of solifluction deposits known as head are also Pleistocene in age. The head deposits sometimes show evidence of frost heave (cryoturbation).",18167,27084,27743,27752,27824,27879&xhr=t&q=Crousa+down&cp=11&pf=p&sclient=psy&aq=f&aqi=&aql=&oq=&gs_rfai=&pbx=1&fp=1aac15d7857edbcb

From Natural England:  "At St Erth in west Cornwall and Crousa Down on the Lizard, there are small outcrops of sand and clay of possible Tertiary age. The St Erth Beds comprise sands and clays which have yielded marine molluscs of Pliocene, or early Pleistocene age (1.5-2 million years old) age. These sediments were deposited in a shallow sea that covered this part of south-west Britain and which cut an erosional platform into the surrounding landmass. Subsequent uplift of the land has led to this platform being located at a present-day height of about 130m".

Not far away we have the Three Brothers of Grugith -- three large boulders making up a cist grave or simple dolmen:

Then there are the Crousa Common menhirs -- apparently made of granite.  Are they erratics?

and of course the famous Giant's Rock at Porthleven is not far away.  That is generally considered to be a glacial erratic, and there has been endless discussion about whether it came from real glacial deposits or was dumped by a passing iceberg or slab of floe ice.......

Does anybody have more info?  Worth investigating further......

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