THE CONTINENTAL GLACIERS OF THE "ICE AGE" 311 



striated. In proportion, however, as the materials are much 

 worked over by the water, the angles between pebble faces be- 

 come rounded and the original shapes considerably masked. 



Features into which the drift is molded. Though the pre- 

 existing valleys were first filled in by drift materials, thus reducing 

 the accent of the relief, a continuation of the same process resulted 

 in the superimposition of features of characteristic shapes upon 

 the imperfectly evened surface of the earlier stages. These 

 features belong to several different types, according as they were 

 built up outside of, at and upon, or within the glacier margin. 

 The extra-marginal deposits are described as outwash plains or 

 aprons, or sometimes as valley trains; the marginal are either 

 moraines or kames; while within the border were formed the till 

 plain or ground moraine, and, locally also, the drumlin and the 

 esker or os. These characteristic features are with few exceptions 

 to be found only within the area covered by the latest of the ice 

 invasions. For the earlier ones, so much time has now elapsed 

 that the effect of weathering, wash, and stream erosion has been 

 such that few of the features are recognizable. 



Marginal and extra-marginal features are extended in the direc- 

 tion of the margin or, in other words, perpendicular to the local 

 ice movement; while the intra-marginal deposits are as note- 

 worthy for being perpendicular to the margin, or in correspondence 

 with the direction of local ice movement. Each of these features 

 possesses characteristic marks in its form, its size, proportions, 

 surface molding and orientation, as well as in its constituent 

 materials. It should perhaps be pointed out that the existing 

 continental glaciers, being in high latitudes, work upon rock ma- 

 terials which have been subjected to different weathering processes 

 from those characteristic of temperate latitudes. Moreover, the 

 melting of the Pleistocene glaciers having taken place in relatively 

 low latitudes, larger quantities of rock debris were probably released 

 from the ice during the time of definite climatic changes, and hence 

 heavier drift accumulations have for both of these reasons resulted. 



Marginal or " kettle " moraines. Wherever for a protracted 

 period the margin of the glacier was halted, considerable deposits 

 of drift were built up at the ice margin. These accumulations 

 form, however, not only about the margin, but upon the ice sur- 

 face as well ; in part due to materials collected from melting down 



