GLACIAL SOILS 59 



and there were also ponds and lakes, some of the latter 

 of very large extent. This water further assisted in 

 moving the ice debris. Such deposits are called modi- 

 fied drift, or aqueo-glacial deposits. For this reason, 

 they have in part been included with glacial soils. 

 The streams, ponds and lakes associated with the ice 

 have given rise to much stratified material, and these de- 

 posits are intimately related in many ways to the purely 

 ice deposits. Beds of gravel, sand and clay are frequently 

 found, and so intimate is their relation to the purely 

 ice deposits that they are sometimes, though incor- 

 rectly, classed with them. These deposits of modified 

 till generally rest upon the distinctly ice deposits, and 

 are of large extent. Around the Great Lakes and in the 

 large valleys of New York and New England, in the 

 valley of the Red River of the North, and in many 

 other places in the Central States, are large areas of 

 such stratified glacial material, ranging in fineness 

 from heavy clay to coarse gravel. These materials 

 constitute some of the most valuable agricultural 

 lands of the country. The Great Lakes region is notably 

 productive, and the Red River Valley of the North 

 is celebrated for its production of small grains. 



The thickness of glacial deposits varies greatly. 

 Pre-glacial valleys may be filled in, and the evidence 

 of their presence completely obliterated. In general, the 

 topographic effect of glacial action is to level the surface. 

 However, in the New England states, where the country 

 is very mountainous, the rocks very hard and the pre- 

 glacial soil blanket meager, the present soil covering is 

 generally thin and very stony. 



