92 STUDIES, SCIENTIFIC AND SOCIAL chap. 



may not be yet able to explain fully how a glacier thus 

 moves, or what slope of the upper surface is required 

 in order that the bottom of the ice may be forced up 

 a given ascent, but the fact of such motion cannot any 

 longer be denied. 



The facts thus established by directed observation of 

 existing ice-sheets, render it more easy for us to accept one 

 of the latest conclusions of British glacialists. A great 

 submergence of a large portion of the British Isles during 

 the glacial period, or in the interval between successive 

 phases of the glacial period, has long been accepted by 

 geologists, and maps have been often published showing 

 the small group of islands to which our country was then 

 reduced, the supposed subsidence being about 1,400 feet. 

 The evidence for this is the occurrence, at a few isolated 

 spots, of glacial gravels containing marine shells in tolerable 

 abundance, the most celebrated being at Moel Tryfaen on 

 the west side of Snowdon, at a height of more than 1,300 

 feet. Shell-bearing drifts have also been found near 

 Macclesfield at a height of over 1,100 feet, and to the east 

 of Manchester at between 500 and 600 feet elevation. 

 Others have since been found on Gloppa, a hill near 

 Oswestry. The fact that the shell-bearing gravels of Moel 

 Tryfaen are nearly 40 feet thick shows that, if they are due 

 to submergence, the land must have remained nearly 

 stationary at that level for a considerable period of time, 

 and there would probably be other stationary periods at 

 lower levels. Yet nowhere in the valleys or on the hill 

 slopes of Wales, or the Lake District, or in the English 

 lowlands are there any of the old beaches or sea cliffs, or 

 marine deposits of any kind, that must have been formed 

 during such a long continued submersion, and which can 

 hardly have been everywhere cleared away by subsequent 

 glaciation. Another difficulty is, that the shells of these 

 drifts are such as could not have lived together on one 

 spot, some being northern species, others southern, some 

 frequenting sandy others muddy bottoms, some which live 

 only below tidal water, while others are shore species. And, 

 lastly, they are very fragmentary, only a small percentage 

 of entire shells being found. 



