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 surface 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 the 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, like 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 
