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Since 1956, when a nuclear spin magnetometer was first developed for towing behind a ship, measurements of the earth’s total field in the western English Channel have been made on six expeditions of R.R.S. Discovery II and R.V. Sarsia. There are now enough profiles to allow a magnetic contour map of the area to be drawn.
The most striking feature of the map is an anomaly with its peak value approximately 15 miles south of the Eddystone and with a total area of 30 x 12 square miles. The discovery of this anomaly has necessitated a reappraisal of the underlying geology which had been previously postulated from coring and seismic results.
Sharp magnetic anomalies to the east of the Lizard show good correlation with the basic intrusive rocks on land but the limited extent of the anomalies to the east appears to disprove the idea of a continuous Lizard-Dodman-Start thrust. The boundary of New Red Sandstone outcrops on the sea-floor, postulated from previous coring results, shows a remarkable coincidence with the extent of the sharp anomalies off the Lizard and off the Start peninsula.
