Seismic investigations of the rocks beneath the northern part of Cardigan Bay, Wales

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Abstract
Griffiths, King & Wilson (1961) described geophysical work in Tremadoc Bay resulting in the recognition of three layers of differing seismic velocities. The thickness and velocity of the uppermost, 'layer 1', and the velocity of 'layer 2' beneath it have now been determined more completely by further seismic refraction observations. Layer 1 has been shown to underly Morfa Harlech in the form of a valley, being the southward extension, some 1000 ft deep, of the Glaslyn valley at Portmadoc. This channel probably continues across Morfa Dyffryn and farther to the south-west. It is filled with material having seismic velocities in the range 5000 to 7600 ft/s, the upper 250 ft of which are thought to be of post-glacial age. The lower part may be Pleistocene material, but is more likely to be older. The seismic velocities found for layer 2 have been compared with measurements made on nearby Ordovician and Cambrian sediments and with results from the Permo-Trias and Carboniferous of the Midlands. This new evidence supports the hypothesis of the previous paper that the northern part of Cardigan Bay is underlain (beneath the more recent cover of layer 1) by Ordovician rocks.