The BGS Lexicon of Named Rock Units — Result Details

Crousa Gravels

Computer Code: COGR Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Pliocene Epoch (NP) — Pliocene Epoch (NP)
Lithological Description: Comprises rounded pebbles of white quartz, 2.5 to 15cm in diameter, uncommon dark grey or blue quartz-tourmaline rock and rare granite, which is very weathered. Near the base of the deposit, the gravel is stratified and contains thin interrupted intercalations of yellow sand and white clay. Towards the top of the deposit, the gravels are disturbed and bedding planes are commonly curved, probably as a result of freeze-thaw action.
Definition of Lower Boundary: An unconformable contact with the underlying gabbro of the Lizard Series, which here is weathered and covered with rounded boulders of less weathered gabbro. Gravel of white and dark grey quartz pebbles, with tourmalinite and granite in the lowest part of the Crousa Gravels are stratified, and contain discontinuous layers of sand and clay.
Definition of Upper Boundary: The present day erosion surface landscape is locally covered by thin Head deposits. Vegetation growing on the Crousa Gravels is typically a scanty growth of heather and thorn.
Thickness: To 6.1m
Geographical Limits: The Lizard Peninsula, Cornwall.
Parent Unit: Not Available (!)
Previous Name(s): Crousa Down Gravel Beds (-3316)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Crousa Down, around Crousa Common, as a series of pits spread over an area of about a square kilometre, which are presently overgrown. Flett, 1946. 
Reference(s):
Flett, J S. 1946. Geology of the Lizard and Meneage. Memoir of the Geological Survey of Great Britain. 
Budge, E. 1843. On diluvial action, as exemplified in the gravel beds and Sienitic Formation of Crousa Down, in the parish of St Keverne. Transactions of the Royal Geological Society of Cornwall, Vol.6, 91-98. 
Reid, C. 1890. The Pliocene deposits of Britain. Memoir of the Geological Survey. 
Campbell, S, Hunt, C O, Scourse, J D and Keen, D H. 1998. Geological Conservation Review Series: Quaternary of South-West England. London: Chapman and Hall. 
De La Beche, H T, 1939. Report on the geology of Cornwall, Devon and west Somerset. Memoir of the Geolgoical Survey. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E359