November, 191 1. The hish Naturalist. 189 



A HIGH LEVEL DEPOSIT OF MARINE SHELLS 

 IN CURRAUN ACHILL, COUNTY MAYO. 



BY J. DE W. HINCH. 



The occurrence of large quantities of marine shells above 

 present high-water mark is often regarded as evidence that 

 at some past time the sea stood at a higher level than at 

 present — and with certain reservations the evidence may 

 be regarded as sound. When we find a formation of water- 

 worn pebbles and of marine shells, of the types present in 

 the adjoining sea at the present day, stretching for con- 

 siderable distances along the present coast-line and at 

 the same general level above present high -water mark, we 

 may regard the evidence as sufficient to warrant the theory 

 that within recent time the sea has stood at the level of 

 the raised beach. There are, however, cases in which the 

 presence of sea-shells at considerable elevations does not 

 necessarily prove that the sea has changed its level, and 

 the explanation must be looked for in another direction. 

 One most common case, especially along the eastern coast of 

 Ireland, is the occurrence of shells in the Glacial drifts. 

 These beds, occurring from sea-level to over 1,200 feet 

 in Ireland and Wales, often contain large numbers of shell 

 fragments, and as many as 37 species have been obtained 

 from the Dublin hills. These shells have been brought 

 to their present position by glaciers intruding on the land 

 from the direction of the sea, and are not evidence of change 

 of sea-level. Their Glacial origin may be confirmed by 

 the appearance of a number of Arctic and northern forms, 

 Leda pernula, Astarte horealis, etc., species which do not 

 occur along our existing coast-lines. A second deceptive case 

 which has to be guarded against is the storm-beach which 

 may be found in favourable localities above sea-level, and 

 which unless supported by collateral evidence such as a 

 definite beach-line cut in rock or Boulder-clay, must be 

 regarded as the result of wave-action working at present 

 sea-level. A third type, which is the one I propose to 

 deal with more fully, has the following characteristic. 



