534 THE JOURNAL OF GEOLOGY. 



sion resembling the kames. A part of these pitted plains seem 

 to be intimately connected in origin with the ice edge and to be 

 due to marginal conditions, among which it has been thought 

 that the incorporation of ice fragments, the grounding of ice 

 blocks, the movement of the ice edge, and the development of 

 underground ice sheets were among the special agencies, but the 

 full explanation of the pitted plains can scarcely be claimed to 

 have been reached. 



III. Formations produced by glacial waters after their issuance 

 from the Pleistocene glaciers. 



i . By glacial rivers. 



One of the most familiar facts of glaciology is the detritus- 

 laden condition of the icy streams as they issue from the body 

 of an active glacier. A portion of this material is thrown down 

 immediately at the margin under the special conditions there 

 presented and constitutes the formations classified above, but 

 the larger portion is borne onward to varying distances and 

 deposited quite independently of the agency of the ice. Two 

 varieties of this class of deposits are worthy of being specially 

 recognized. 



(i). " Valley drift." — In those cases in which the previous 

 surface agencies had developed a definite drainage topography, 

 and in which the gradient was favorable, the detritus was borne 

 down the valleys leading away from the ice border in trains of 

 gravel and sand. As the glacial streams were usually greatly 

 overloaded with detritus at the outset, they built up their valley 

 bottoms by depositing material from bluff to bluff constituting a 

 valley plain, out of which subsequently beautiful systems of 

 terraces were often cut. The most notable class of this type 

 consists of those gravel valley-plains which head in a terminal 

 moraine or, more strictly speaking, in the overwash apron of a 

 terminal moraine. Sometimes the apron gathers in for many 

 miles on either side giving a very broad expanded head to the 

 valley tract. As these valley tracts may. head on successive 

 moraines and may be traceable far clown their valleys, they afford 



