286 THE ICE AGE AND ITS WORK. 



been accepted by geologists, and maps liave been often published 

 showing- tlie 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 spots of glacial gravels containing 

 marine shells in tolerable abundance, the most celebrated being at Mod 

 Tryfaoi, on the west side of Snowdon, at a height of more than l,30l> 

 feet. Shell- bearing drifts have also been found near Macclestield at a 

 height of over 1,100 feet, and to the east of Manchester at between 500 

 and 000 feet elevation. Others have since been found on Gloppa,a hill 

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

 faen are nearly 40 feet thick shows that, if they are due to submerg- 

 ence, the land must have remained stationary at that level for a con- 

 siderable iieriod 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 beaclies or sea cliffs, or marine deposits of any kind, that 

 must have been formed during such a subsidence and which can hardly 

 have been everywhere cleared away by subsequentglaciation. 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. - - - 



11. EROSION OF LAKE BASINS. 



Lakes are distributed very unequally over the various parts of the 

 world, and they also differ nuich m their position in relation to other 

 j)hysical peculiarities of the surface. Most of the great continents 

 have a considerable number of lakes, many of great size, situated on 

 jilateaus or in central basins ; while the northern parts of Europe and 

 North America are thickly strewn with lakes of various dimensions, 

 some on the plains, others in sub-alpine valleys, others again high up 

 among the mountains, these latter being of small size and usually 

 called tarns. The three classes of lakes last mentioned occur in the 

 greatest profusion in glaciated districts, while they are almost absent 

 elsewhere; and it was this peculiarity of general distribution, together 

 with the observation that all the valley lakes of Switzerland and of 

 our own country occurred in the track of the old glaciers, and in situa- 

 tions where the erosive power of the ice would tend to form rock closed 

 basins, that appears to have led the late Sir Andrew Eamsay to formu- 

 late his theory of ice erosion to explain them. He was further greatly 

 influenced by the extreme difficulty or complete inadequacy of any pos- 

 sible alternative theory, — a difficulty which we shall see remains as 

 great now as at the time he wrote. 



This question of the origin of the lake basins of the glaciated regions 



