Area: 79.73 hectares.
The site contains two Geological Conservation Review sites. The site is in Cornwall Area of Outstanding Natural Beauty.
Description and Reasons for Notification:
This site covers the deposits of the Gramscatho Group and their contact with the Lizard complex. The latter is a large serpentinised peridotite body largely enclosed by amphibolites and cut by later gabbros, basic dykes and granite veins. Recent interpretations consider the complex to represent the tectonically juxtaposed remnants of a disrupted ophiolite unit, with the deformation within the Gramscatho Group being formed in part as a response to the northward thrusting of the ophiolite during the Hercynian (Variscan) Orogeny .
The Devonian Gramscatho Group is one of the very few examples of a large scale subaqueous sedimentary slump deposit (olistostrome) in Great Britain. The site also includes exposures of Ordovician and Silurian rocks (preserved as olistholiths).
Excellent exposure along the coast between Porthallow and Porthkerris shows a range of lower crustal rocks, principally mafic and ultra mafic cumulates, forming an imbricate zone close to the northern boundary of the complex. This boundary, at Porthallow, shows the igneous rocks in over thrust contact with the sheared and locally metamorphosed sediments to the north.
This coastal section provides important clues to the geological formation of the Lizard complex, and is the subject of ongoing research.