136 EXCA VATED LAKES. 



lakes has been produced as a consequence of compressions which have 

 thrown the rocks into parallel folds on a smaller scale. Thus valleys 

 have been formed and closed by tilting so as to have no outlet for the 

 drainage. Lakes of this kind are usually situate among mountains, 

 and probably no better examples could be taken than the lakes on the 

 west coast of Scotland, or those of the Alps, which are ancient fiords 

 closed by upheaval. No broad distinction can be drawn between such 

 lakes as these and those of the former class, for both have originated 

 in compressions of the earth's crust. Lakes like Neufchatel may 

 have been fresh ever since the Alps attained their present elevation, 

 although sea-birds still frequent the higher Alpine waters. 



A third group of lakes is exemplified by some in Scotland, which 

 have been described by the Duke of Argyll and others, in which 

 the waters lie along anticlinal folds or saddles, and must be supposed 

 to owe their existence to the ease with which an anticlinal fold was 

 excavated by the sea when the level of the land was lower. 



Excavated Lakes. A fourth group of lakes would appear to have 



Fig. 45. Helm Grag and Grasmere. 



been excavated by the erosive power of glaciers, towards the close of 

 the glacial period, when glaciers descended from all the principal 

 mountains in our own country and Northern Europe. Some of these 

 lakes, like those at the foot of Snowdon, and elsewhere in North 

 Wales, are found to be dammed up at their lower end by loose un- 

 stratified materials, which must be regarded as a terminal moraine. 

 Others of these lakes lie in true rock basins, or depressions excavated 

 in the solid rock, and appear to exist at such places as would be the 

 point of meeting of a great glacier and one of its tributaries. Here 

 the mere increased weight of the ice may be supposed to augment 

 the excavating power of the morainic stones in its floor. Several of 

 the lakes of Cumberland are susceptible of explanation in this way. 

 ' Crater Lakes. Finally, there are lakes which till the craters of 



