The peninsula is dominated by sedimentary rocks of the Devonian and Carboniferous Periods and includes cotemporaneous minor basaltic volcanic rocks. Into these have been intruded Late Carboniferous to Permian age granites and associated subvolcanic rocks. The geology of the region has led to a variety of scenery ranging from dramatic cliffscapes of sedimentary rocks, castellated granite, pillow lavas, granitic tors and drowned valleys. Tectonic plate movement over the past 400 million years has shifted the position of Cornwall and Devon from well south of the equator to its present geographical position located at 50 degrees N.

St. Anthony
Rocky Valley
Porthcurno
Clodgy Point
Bodmin Moor
After Bromley, 1989.

 

Sandstone
Conglomerate
Serpentinite
Limestone

 

The Devonian and Carboniferous sedimentary rocks comprise mudstones,siltstones, sandstones, minor conglomerates and rare limestones as well as basic intrusive (gabbro) and extrusive (spillites and tuffs) volcanic rocks known locally as greenstones.

The Devonian rocks of the region are separated by a broad zone of Carboniferous rocks (the Culm) in central Devon. Offshore lie Permian and Cretaceous sedimentary rocks. At the extreme southeast of the region lies the Lizard peninsula, a fragment of Devonian oceanic floor of mainly ultrabasic rocks, most of which have been altered to serpentinite. Little of the Pre-Devonian basement is observable apart from the Man O’ War Gneiss at the Lizard Point which is dated at 500 million years (Ma), Ordovician quartzites in the Roseland Breccia formation and rare xenoliths in the basaltic rocks.

Hand specimen and thin section of rock types from the Cornubian Orefield Region can be seen by downloading this PDF file (3.42MB) or for world rocks and textures see this link.

 
Gneiss
Siltstone
Breccia
Gabbro

 

Folding
Delabole 'butterfly'
Delabole slate quarry
The sediments have been deformed (folded) both prior and during the period of granite intrusion with later minor deformation. Most of the Devonian sediments have undergone low-grade regional metamorphism to sub-greenschists (development of chlorite) facies. Slates formed during this period are now exploited at Delabole. Occasionally fossil remains of the Cyrtospirifer verneulli (‘Delabole butterfly’) are found in the quarry. The major period of deformation took place during the Variscan orogeny (mountain building). It was in the latter part of this orogeny that the granites were intruded with associated tin and copper mineralization, forming the Cornubian Orefield.
Permian sandstone
China clay pit
Ballclay pit
Raised beach and head

During the Permian to Cretaceous deposition again took place in basins developed on Devonian and Carboniferous deformed rocks, forming sandstones and conglomerates with minor volcanic rocks and later chalk. The only exposures near the Cornubian Orefield are the New Red Sandstones to the northeast in Devon, the rest lie offshore. During the Tertiary, fault activation resulted in basins now evident as isolated Oligocene, Miocene and Pliocence sands and clays and deep weathering of the granite to form kaolin deposits. During the Quaternary the Ice sheet probably touched the north coast of the peninsula and Isles of Scilly and the area was subjected to periglacial acivity.It was during this period that sea-level changes lead to raised beaches and solifluction formed head deposits. Postglacial sea level rise produced drowned valleys or rias forming deep estuarine inlets.

Riated valley

A summary of the geological history can be seen in the Time Line...

After Bristow, 1996.

 



 

This site was last updated on Monday, January 15, 2007 3:50 PM .
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