194 Frank Carney 



DEPOSITION BY GLACIERS 



Glacial deposits are always heterogeneous in material, and 

 usually so in texture and structure. The word "drift" includes 

 all debris transported and deposited by glaciers or streams issuing 

 from glaciers. ''Till" refers to the deposits made directly from 

 the ice. The material assorted by glacial waters is called ''modi- 

 fied drift." The designation "drift," therefore, includes the other 

 two. 



The simplest condition of glacial accurnulations. The earliest 

 terms used in reference to the accumulations of glaciers referred 

 particularly to valley glaciers. Valley glaciers carry along their 

 sides much material rasped from the walls of the valley. At 

 their ends, where melting takes place, the debris not carried away 

 by the outwash stream accumulates, forming a "terminal mor- 

 aine." The deposits that gather along the sides or margin of a 

 glacier are called "lateral moraines." When one valley is tribu- 

 tary to another and a glacier occupies each, the two lateral 

 moraines, below the point of coalescence, unite, forming in the 

 single glacier strearri a "medial moraine." These three designa- 

 tions for moraines are the earliest and best fixed in the literature. 

 Unfortunately, their limited use has given rise to much miscon- 

 ception in America, where only a fraction of the ice deposits was 

 made by valley glaciers. 



The more complex conditions of a continental ice sheet. The 

 deposits made by a continental glacier are of a very complex 

 origin. The drift which we study to-day, in most of the states, 

 was left by the retreating ice sheet. This retreat was slow. All 

 the conditions of melting and drainage along its front merely 

 repeated conditions that obtained while the ice sheet was expand- 

 ing. If, after the continental glacier had attained its maximum 

 growth, the conditions that induced glaciation had suddenly 

 ceased to operate, and all of the ice had melted in situ, as an ex- 

 posed block would melt on the sidewalk, the glacial deposits 

 made would be simple. A terminal moraine accumulated as 

 often as the advancing ice sheet held a stationary position for any 

 great length of time; when the glacier advanced further, this 

 moraine was subject to ice erosion. For this reason, much of the 

 moranic material, which accumulated as the ice sheet was spread- 

 ing out over the country, was altered. For this reason, the drift 



