GEOLOGICAL HISTORY OF THE EARTH 4! 



Figure 7), which extended from the south border of the 

 receding glacier above Winnipeg, Canada, to the region 

 of Lake Traverse on the western side of Minnesota, 

 where it had its outlet through what is now called the 

 Minnesota river, deposited in its basin a final surface 

 layer of fine clay, covering what is now known as the 

 valley of the Red River of the North. Clay soils, thus 

 laid, have their good qualities and their disadvantages, as 

 compared with the best types of mixed soils. 



An undulating country. The drift formed a generally 

 undulating surface over the upper Mississippi valley 

 region, the flood waters having eroded hollows, giving 

 natural surface drainage ; and the gentler forces acting 

 through the long ages since the glacial period have 

 rounded down any steeply washed banks, making the 

 country one of beautiful broad hills and vales. The 

 great prairies of the West resemble the swells of a high 

 sea, with the waves and troughs much enlarged. 



In digging into the drift, as in making wells, layers of 

 clay, sand, gravel and unassorted till are met with. 

 Sometimes these seem to have been deposited in an 

 unnatural order, or lie in two or more series of layers. 

 In some places glacial streams have eroded large 

 masses of the drift, and, carrying it forward, left it in 

 assorted layers. Too often sandy or gravelly layers 

 have been left at the surface, and form poor soils and 

 subsoils. In other cases the unassorted till forming the 

 surface contains bowlders, which are an impediment to 

 cultivation. There are areas within the glacial region 

 over which the glacier did not flow; some are covered 

 with soils formed from easily decaying rocks, or, as 

 north of Lake Superior and in mountainous regions, are 

 of granite, trap rock or sand rock uncovered with soil; 

 others were formed as clay layers and rock ledges. The 

 forces still acting on the soil cause it to become more 

 productive; the action of water, of bacteria, of plant 



