68 J. Croll — Evidence of former Glacial Periods. 



which that land-surface may once have presented. We cannot, 

 for example, expect to meet with polished and striated stones 

 belonging to a former land glaciation ; for such stones are not 

 carried down bodily and unchanged by our rivers and deposited 

 in the sea. They become broken up by subaerial agencies into 

 gravel, sand, and clay, and in this condition are transported 

 seawards. Even if we supposed it possible that the stones and 

 bowlders derived from a mass of till could be carried down to 

 sea by river action, still these stones would certainly be 

 deprived of all their ice-markings, and become water-worn and 

 rounded on the way. Professor James Geikie states that the 

 great accumulations of gravel which occur so abundantly in the 

 low grounds of Switzerland, and which are, undoubtedly, 

 merely the re-arranged materials originally brought down' from 

 the Alps as till and as moraines by the glaciers during the 

 Glacial period, rarely or never yield a single scratched or 

 glaciated stone. The action of the rivers escaping from the 

 melting ice has succeeded in obliterating all trace of striae. It 

 is the same, he says, with the heaps of gravel and sand in the 

 lower grounds of Sweden and Norway, Scotland and Ireland. 

 These deposits are evidently in the first place merely the 

 materials carried down by the swollen rivers that issued from 

 the gradually melting icefields and glaciers. The stones of 

 the gravel derived from the demolition of moraines and till, 

 have lost all their striae and become in most cases well rounded 

 and water- worn. Further, we cannot expect to find bowlder 

 clay among the stratified rocks, for bowlder clay is not carried 

 down as such and deposited in the sea, but under the influence 

 of the denuding agents becomes broken up into soft mud, clay, 

 sand, and gravel, as it is gradually peeled off the land and 

 swept seawards. Patches of bowlder clay may have been now 

 and again forced into the sea by ice and eventually become 

 covered up ; but such cases are wholly exceptional, and their 

 absence in any formation cannot fairly be adduced as a proof 

 that that formation does not belong to a glacial period. 



It may, however, be replied that there is one kind of evi- 

 dence of former glacial periods which we ought to expect in 

 the stratified rocks, viz : the presence of large erratic blocks 

 embedded in strata which, from their constitution, have 

 evidently been formed in still water. But even allowing this 

 to be the case we cannot regard the absence of such blocks as 

 proof that no glacial period occurred during the time of the 

 formation of the strata ; for their mere absence may be the 

 indication either of a period of extreme glaciation, or a period 

 absolutely free from ice. This absence is a result which 

 would as truLy follow from the former condition of things as 

 from the latter. Glaciers carry erratic blocks on their surfaces, 



