THE WORK OF SNOW AND ICE 



125 



running water, for in the latter the depressions have outlets, and 

 the hills and ridges stand in a very definite relation to the valleys. 



Fig. 132. One phase of ground moraine topography; elongate hills of 

 drift of the type shown here are called drumlins; southeastern Wisconsin. 

 (U. S. Geol. Surv.) 



In valleys, terminal moraines often make dams, and so pond the 

 waters of the streams above, making lakes. The basins of many of 

 the lakes of the glaciated mountain valleys were made in this way. 



Fluvio-glacial Deposits 



Water flows in abundance from all glaciers in the summer, and 

 from many glaciers all the time. Stream-work, therefore, accom- 

 panies glaciation in all cases, and some of the drift left by ice is 

 modified by water afterward. 



The streams which flow from glaciers carry much sediment. At 

 the outset, this sediment consists of both coarse and fine material, 

 but the coarse materials are generally dropped before the water has 

 flowed far. The sand is carried farther than the gravel, and the 

 silt farther than the sand. Many streams flowing from glaciers 

 carry so much silt in suspension that the water is turbid, and if the 

 silt is whitish, as it often is, the streams are said to be " milky." 



