Cardigan Bay occupies a considerable portion of the west coast of Wales. It is bounded on the north by the southern shores of Carnarvonshire; its central portion comprises the entire coast-lines of Merioneth and Cardigan, and its southern limit is the north coast of Pembrokeshire. The total length of coast-line between Braich-y-pwll in Carnarvon, and Strumble Head in Pembrokeshire, is about 140 miles, and in addition there are considerable estuarine areas. The entire Bay is shallow; for the most part four to ten fathoms inshore, and ten to sixteen about the centre. It is considered probable that the Bay was temporarily transformed into low-lyingland by accumulations of boulder clay during the Ice Age. Wave action has subsequently completed the erosive removal of that land area, with the exception of a few patches on the present coast-line and certain causeways or sarns. Portions of the sea-floor probably still retain some remains of this drift, and owing to the shallowenss, tidal currents and wave disturbance speedily cause the waters of the Bay to become opaque. The prevailing winds are, as usual, south-westerly, and heavy surf is frequent about the central shore-line. This surf action is accentuated by the large amount of shingle derived from the boulder elay. The action of the prevailing winds and set of dirfts in the Bay results in the constant movement northwards along the shroes of a very considerable quantity of this residual drift material. Where checked by shore contour or river current this drift accumulates to form storm-beaches, which have eventually deflected certain rivers and streams to the northward, as in the case of the Ystwyth at Aberystwyth, the Clarach stream a little further to the north, and the Leri at Borth.