LAKES 149 



derground movements may have originated certain of 

 our lakes, or at least may have fixed the direction in 

 which they have otherwise been produced. 1 



A large number of British lakes lie in basins of 

 hard rock, and have been formed by the erosion and 

 removal of the solid materials that once filled their 

 sites. The only agent known to us by which such 

 erosion could be effected is land-ice. It is a significant 

 fact that our rock-basin lakes occur in districts which 

 can be demonstrated to have been intensely glaciated. 

 The Ice-Age was a recent geological episode, and this 

 so far confirms the conclusion already enforced, that 

 the cause which produced the lakes must have been 

 in operation recently, and has now ceased. We must 

 bear in mind, however, that it is probably not necessary 

 to suppose that land-ice excavated our deepest lake- 

 basins out of solid rock. A terrestrial surface of 

 crystalline rock, long exposed to the atmosphere, or 

 covered with vegetation and humus, may be so deeply 

 corroded as, for two or three hundred feet downward, 

 to be converted into mere loose detritus, through which 

 the harder undecomposed veins and ribs still run. 

 Such is the case in Brazil, and such may have been 

 also the case in some glaciated regions before the 

 glaciers settled down upon them. This superficial cor- 

 rosion, as shown by Pumpelly, may have been very 

 unequal, so that when the decomposed material was 

 removed, numerous hollows would be revealed. The 

 ice may thus have had much of its work already done 



1 Lough Neagh has been produced by subsidence, probably since 

 the Glacial Period. See Ancient Vokanoes of Great Britain, 

 vol. ii. p. 448. 



