STRATIFIED DRIFT 963 



extraglacial streams were depositing gravel, sand, or silt in the 

 valleys through which they flowed. Wherever the ice halted 

 temporarily in its advance, valley trains of greater or less extent 

 may have been developed. All those valley deposits which were 

 made during the first advance of the ice were made on territory 

 which was free from glacial drift. Subsequently the glacier ice 

 overrode them in whole or in part, often burying them beneath 

 its own moraine deposits (till). So too, during the first advance 

 of the ice, the waters which did not concentrate themselves in 

 valleys as they issued from the edge of the ice, made deposits 

 in the form or in the position of overwash plains of gravel, sand, 

 or silt. Well developed overwash plains may have been built up 

 before the ice reached its maximum extension, wherever the ice 

 edge stood for a sufficiently long interval of time in a favorable 

 topographic position. Such overwash plains as were developed 

 during the first advance of the ice, lay upon territory which the 

 ice had never invaded, and constitute, if they still remain, the 

 lowest member of the drift series. Subsequently, in its further 

 advance, the ice overrode these deposits sometimes destroying 

 them and sometimes burying them beneath deposits strictly 

 glacial. 



It was not simply by extraglacial streams that stratified 

 drift was deposited during the advance of the ice. The mar- 

 ginal lakes which came into existence during the advance of the 

 ice, and there were many, likewise gave rise to stratified deposits 

 of glacio-lacustrine origin. So far as these were formed upon 

 territory which was free from drift, and subsequently overridden 

 bv the ice, they were likely to be buried so far as not destroyed. 

 Deposits formed in the margins of seas at the edge of the ice 

 would be subject to the same changes as those formed in lakes, 

 in so far as they were subsequently overridden by the ice. 



Still supposing the edge of the ice not to have oscillated, all 

 the deposits of extraglacial waters made during its first advance, 

 whether of the valley-train, overwash plain, lacustrine or other 

 types, were liable to destruction by the further progress of the 

 ice. So far as they were not destroyed they were liable to bur- 



