964 STUDIES FOR STUDENTS 



ial beneath unstratified drift deposited by the ice itself. So far 

 as not destroyed, therefore, the exisiting deposits of stratified 

 drift made during the first advance of the ice are likely to 

 occupy the basal position (submorainic) in the drift series, in 

 all the territory subsequently overspread by the ice. 



Effect of edge oscillation. — Hitherto the assumptions have 

 been made, for the sake of simplicity, that the advancing edge 

 of the ice forged steadily forward, and that the retreating edge 

 was subject to no temporary advances. It is probable that 

 neither of these assumptions is true. It is believed rather that 

 the advance of the ice was interrupted by many minor oscilla- 

 tions of its edge, both seasonal and periodic, though the sum of 

 the advances was greater than the sum of the retreats during any 

 given epoch, up to the time when the ice reached its greatest 

 extension. When the ice advanced to a certain line, and then 

 receded temporarily, incipient overwash plains, or valley trains, 

 or lacustrine beds, or ill-defined patches of gravel and sand, were 

 doubtless deposited on the territory from which the ice had tem- 

 porarily receded. The gravel and sand in this case would in 

 general lie, not on a driftless bed, but over deposits made by the 

 ice before its temporary recession. The subsequent advance of 

 the ice would be likely to bury these deposits of stratified drift 

 so far as it did not destroy them. Thus by oscillations of the 

 edge of the ice during the general period of its advance, strati- 

 fied sand and gravel may have come to be enclosed between 

 beds of till. The extent of the area where this sort of action 

 might take place at any one time would depend upon the 

 amount of oscillation which the ice underwent during its 

 advance. But it may have taken place at many times and places 

 and at many stages in the development of an ice-sheet, so that 

 the interbedding of the two types of drift by this process may 

 have been considerable, in the aggregate, during the advance of 

 the ice. 



Deposits jnade by extraglacial ivaters duri?tg the retreat of the ice. 

 — Stratified deposits made by extraglacial streams during the 

 retreat of the ice of any epoch would remain at the surface 



II 



