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A Section across the Lizard Peninsula

Around 390 million years ago, due to continental drift, dense oceanic rocks of the Normannian tectonic plate partially subducted under the lighter continental crust of the Laurasian plate to the north. The resultant convection of magma and major earth movements produced three distinct zones in the area. The collision happening in such a way that rocks usually only found in the mantle, below the moho, namely serpentine, are visible today at the earth's surface. The rocks off the southern coast at Lizard Point are highly altered remnants of the oceanic Normannian plate, the mid-section of the Lizard is a zone of various metamorphic and igneous rocks formed in a variety of ways, the northern section consisting chiefly of Devonian slates altered to a greater or lesser extent depending on their proximity to the actual crush zone. There are a couple of excellent reference books on the area: Geology of Cornwall by Selwood, Bristow and Durrance, and Earth Story: The Shaping of Our World. More information on the Variscan Orogeny itself can be found at link (requires a .pdf reader).


Simplified map of the geology of the Lizard Complex

Taking a North-South section across the Lizard, the following sequence would be encountered:-


NORTH

  • Location: Northern Boundary of the Peninsula Rock Type(s) Volcanic Ash Notes A thin layer of volcanic ash indicating the turbulent history and formation of this whole area.

  • Location: North of Loe Bar Rock Type(s) Sedimentary Rocks and the Mylor Slate Formation Notes Overlying the flint layer lie sediments washed down by ancient rivers, from the erosion of the Laurasian plate to the north.

  • Location: Loe Bar Rock Type(s) Flint and Chert Notes The remains of microscopic animals known as 'radiolaria' - their silica skeletons eventually form into the flint and chert that is so common on this beach.

  • Location: Jangye-Ryn (Dollar Cove),Gunwalloe Rock Type(s) Devonian Slates of the Portscatho Formation. Notes The slates first met at Poldhu Cove have been deformed and weakened by the intense pressure involved and now form a series of coves and inlets with low cliffs. At Jangye-Ryn, slate strata can be seen drastically folded and contorted; however, the further away from the metamorphic zone the less evidence of deformation there is.

  • Location: North of Porthallow Rock Type(s) Meneage Breccia Notes Jumbled rock debris- the 'waste' bulldozed up ahead of an advancing continent.

  • Location: Polurrian Cove Rock Type(s) Hornblende Schists Notes This metamorphic rock contains micas and other platy minerals, aligned to give a foliation. Some schists 'garnet mica schists' also contain the semi-precious mineral garnet.

  • Location: Mullion Island Rock Type(s) Pillow Lavas Notes Rapidly cooled, hence fine-grained igneous volcanic rock from the mantle erupting onto the ancient sea floor.

  • Location: Manacle Point Rock Type(s) Basalt (Dykes) Notes The fractured oceanic crust has been intruded with dykes of basalt and greenstone. The dykes were essntially the 'feeder pipes' of the overlying pillow lavas.

  • Location: Porthoustock & Coverack Rock Type(s) Gabbro Notes Overlying the serpentine and cropping out on the southeast of the Lizard, there is a thick layer of this coarse grained igneous rock. Once again coarse graining indicate solidification at depth. Being quarried at Dean Quarry

  • Location: Kynance Cove Rock Type(s) Serpentine Notes Rocks of the mantle - normally not seen at the surface. Forming over half the area of the Lizard Peninsula. The serpentine must have solidified at depth before being thrust upward by tectonic plate movements.

  • Location: Man of War Reef (south of Lizard Point) Rock Type(s) Quartzite Notes Highly altered rocks of the Normannian Plate - similar to rocks found in Brittany and Normandy.

SOUTH
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