Ch. 1] ICE 13 



wind changes, the inclination and direction of the layers of sand like- 

 wise change. Consequently dune deposits are characterized by cross- 

 bedding. 



Ice 



Extensive deposits of glacial sediments are found in northern lati- 

 tudes, in both North America and Europe. These deposits consist of 

 three principal types: (1) groundtill, (2) morainal material, and (3) 

 outwash. As the glacier moves along, it plucks up rock fragments and 

 boulders of all sizes. In addition it scours the rocks over which it 

 passes, producing much fine debris of silt and clay size. This frag- 

 mental material becomes embedded in the ice, and, when the ice 

 melts, the debris settles on the surface of the land, forming deposits 

 called groundtill. The thickness of this till ranges from a few feet 

 to several hundred feet, depending to some extent upon the depth and 

 past history of the ice mass. This till covers the ground like a blanket 

 and is composed of particles of all conceivable sizes, from huge bould- 

 ers many feet in length to finely comminuted clay. The rock fragments 

 are angular in outline but commonly have rounded edges. The par- 

 ticles of clay size in some areas contain relatively little mineral clay. 

 Thus the stability of the till is likely to be materially different than 

 that of normal fine-grained deposits of lakes, rivers, or the sea, a 

 point foundation engineers should bear in mind. An essential char- 

 acteristic of till is the presence of numerous impermeable layers or 

 zones which prevent the free flow of water. 



A similar type of deposit called morainal material is laid down at" 

 the sides or in front of glaciers. The front of a glacier is maintained 

 by a balance between the rate of melting of the ice and the rate of 

 forward movement of the glacier. Commonly this balance is so even 

 that the front of a glacier stays at one position for a long time. As 

 the ice is constantly moving, the material carried with it is trans- 

 ported to the front of the glacier and is dumped when the ice melts. 

 Thus big ridges called moraines are built up. Similar ridges form at 

 the sides of the mass of moving ice. The material in these moraines 

 is similar to groundtill. 



Water-laid deposits also are associated with glaciers. The ice 

 melts during summer, and the melted water finds its way along the 

 surface of the ice, in places dropping down cracks to the interior or 

 bottom. Eventually this water emerges from the side or front of the 

 glacier and deposits its load in broad plains. As the water flows across 

 these plains rapidly, the deposits are well sorted, and many of the 

 particles of gravel size are well rounded. 



