GEOLOGICAL HISTORY OF THE EARTH 39 



operation, and when the ice melted this material was 

 deposited along the margin of the glacier. Where the 

 recession of the ice sheet was gradual, the debris was 

 laid out in rather level sheets over the surface of the 

 earth and is now called by the geologists drift or till. 

 In some places this material still lies as it was deposited, 

 with fine and coarse particles mixed, but in many places 

 the water flowing from the melting glacier washed the 

 drift about, often making great valleys through which 

 the streams could run. The material which was thus 

 washed about by the water, was usually left assorted 

 and in layers, just as the stream on the hillside carries 

 forward gravel and small stones, particles of sand and 

 clay, depositing the heaviest particles first. The finer 

 particles of clay and silt are often carried long dis- 

 tances before they are deposited in the quiet waters of 

 some lake or large stream. Since the fine particles were 

 carried out in the water beyond where the coarser par- 

 ticles were deposited, the clay is usually found above the 

 stones, gravel and sand. 



The formation of morainic hills. In some instances, 

 owing to temporary cessation in the increasing warmth, 

 the glacier, instead of receding with regularity, stopped 

 for a time, or even again progressed for a period. The 

 melting taking place only as rapidly as the ice flow 

 proceeded, in case the front edge of the glacier remained 

 stationary, there was naturally much debris left in the 

 place where the ice melted, for the ice always carried 

 within its body stones and finer particles of earth. In 

 this way ranges of hills were formed. These are called 

 moraines, of which there is an example in southwestern 

 Minnesota, called the Coteau Hills, and many others in 

 the states over which the glacier flowed. 



Unassorted till formed good soils. Where the glacier 

 dropped its debris in such a way that we find no distinct 

 laminae or layers, the geologists call it '' till," or " 



