This diving expedition to Cornwall was an innovative geological research project designed to test and develop methods for underwater geological mapping using self‑contained diving equipment. Based between St. Agnes and Cligga Head on the north coast of Cornwall, the expedition aimed to extend geological mapping from land onto the seabed, particularly to trace the offshore continuation of metamorphic aureoles surrounding the St. Agnes and Cligga granites—key structures associated with Cornwall’s tin‑copper mineralization.
A team of six trained diving geologists conducted one month of fieldwork, launching boats daily from Trevaunance Cove. A detailed bathymetric chart—compiled from Admiralty soundings and enhanced by a portable echo‑sounder—guided the selection of underwater targets. Diving operations involved pairs of divers descending to average depths of 55 feet, collecting bedrock samples, recording structure, photographing outcrops, and mapping seabed geology through repeated spot‑dives. Despite poor weather limiting operations to 12 workable days, the team completed 70 location‑dives across Cornwall, with 22 yielding seabed rock specimens in the St. Agnes area.
Results showed that bedrock outcrops cover roughly 40% of the surveyed seabed. A persistent sand belt—up to six feet thick and extending one‑third of a mile offshore—concealed many areas, but beyond this, ridged exposures of “killas” (metamorphosed Devonian slates and grits) were common. Structural measurements revealed variations in strike corresponding with seabed ridges. Crucially, the offshore continuation of the Cligga Granite metamorphic aureole was confirmed for at least one mile out to sea, suggesting a larger extent than inferred from land exposures. No granite outcrops or lodes were detected offshore, though geological reasoning indicates mineralization may continue beneath the seabed.