EXTENSION COURSE IN SOILS, 7 



notable example of alluvial deposit regularly renewed. Large soil 

 areas of this nature are common in the valleys of the Mississippi River 

 and its tributaries, and those of smaller extent are common in the 

 northern United States. 



Glacial soils (Ref, No. 2, pp. 54-61). — In lands far toward the poles 

 snow accumulates to great depths, and its pressure becomes such as 

 to compact it into immense fields of ice. Where sloping land surfaces 

 or valleys occur, the force due to gravity causes these sheets of ice, 

 called glaciers, to move slowly down the inclines, grinding the rock 

 surfaces and carrying along large bowlders and much soil material. 

 When the ice front of winter begins to melt and recede, as summer 

 approaches, there is left a layer of miscellaneous ground rock mate- 

 rials whose position has been more or less affected by the carrying 

 properties of the water formed by the melting ice. Such formations 

 of soil are constantly being produced in the Arctic and Antarctic 

 regions. This condition illustrates a period in recent geological 

 times when immense sheets of ice moved over the land sm'face of the 

 earth, in both Northern and Southern Hemispheres, much beyond 

 the present limits of perpetual snow. Soils formed as the result of 

 the action of glaciers during this ice age are called glacial soils. (Ref. 

 3, p. 52.) In the United States soils of glacial formation extend ap- 

 proximately to the line described as the northern boundary of residual 

 soils, page 5. 



It is easy to understand how th,e character of glacial soils may vary 

 widely even within the limits of small areas, since they are composites 

 of all the rock materials over which the ice sheets have passed. 

 Where the ice moved across granite rocks it mixed the residual soil 

 previously formed from the granite with cobbles and bowlders 

 brought from farther north. The granite rock itself was too hard to 

 be much affected by the ice, though it was often polished quite 

 smooth. On the other hand, when the ice sheets passed over areas 

 underlain by sandstone, which is much softer than granite, the rock 

 was ground up and formed into a sandy soil of rolling topography. 

 The chemical composition of the soil, however, Uke residual soils from 

 the sandstone, was not much changed. The ice in passing over lime- 

 stone country ground up a good deal of the limestone underlying the 

 surface residual soil, mixing it with the surface and forming a soil 

 richer in limestone, or calcium carbonate, than the corresponding 

 residual soil. The glaciers in their movement often filled up valleys 

 and in many cases left shallow basins which filled up with water until 

 an outlet was found. The region which was covered by glacial ice 

 is characterized, therefore, by a large number of small lakes and 

 marshes since formed in lake beds. When the glacial sheets 

 receded, the water flowing from the melting ice carried with it the 

 sedimentary materials ground up in the ice, producing fanlike plains 



