952 STUDIES FOR STUDENTS 



as if interruptions in the advance and in the retreat, respec- 

 tively, had not occurred. 



As they now exist, the deposits of stratified drift made at 

 the edge of the ice or beyond it during the period of its 

 maximum extension present the simplest and at the same 

 time most sharply defined phenomena, and are therefore con- 

 sidered first. 



DEPOSITS MADE BY EXTRAGLACIAL WATERS DURING THE MAXI- 

 MUM EXTENSION OF THE ICE. 



The deposits made by the water at the time of the maximum 

 extension of the ice and during its final retreat, were never dis- 

 turbed by subsequent glacier action. So far as not destroyed by 

 subsequent erosion, they still retain the form and structure which 

 they had at the outset. Such drift deposits, because they lie at 

 the surface, and because they are more or less distinct topo- 

 graphically as well as structurally, are better known than the 

 stratified drift of other stages of an ice-sheet's history. 



Of stratified drift made during the maximum extension of the 

 ice, and during its final retreat, there are several types. Some 

 of them have been adequately described and defined in the lit- 

 erature of glacial geology, and would need no more than pass- 

 ing reference in this connection, were it not that, under certain 

 conditions, they lose their distinctive characteristics, without 

 being altogether destroyed. Their recognition then becomes a 

 matter of difficulty, and their real relations are likely to be mis- 

 understood, when the phenomena are in reality rather simple. 



A. At the edge of ice, on land. — If the subglacial streams 

 flowed under " head," the pressure was relieved when they 

 escaped from the ice. With this relief, there was diminution of 

 velocity. With the diminution of velocity, deposition of load 

 would be likely to take place. Since these changes would be 

 likely to occur at the immediate edge of the ice, one class of 

 stratified drift deposits would be made in this position, in imme- 

 diate contact with the edge of the ice, and their form would be 

 influenced by it. At the stationary margin of an ice-sheet, there- 



