590 THE GLACIAL LAKE AGASSIZ, 



joined in making up tlie drift throiigh the grinding and kneading action of 

 the ice-sheet 



Foregoing descriptions and analyses of the waters of wells, lakes, and 

 streams have sufficiently indicated the effects of this alkaline matter of the 

 soil upon the water supply. At the same time, the result of evaporation 

 from the soil during droughts, often producing on previously moist tracts 

 a saline and alkaline efflorescence, was also noticed. Shallow and flat 

 depressions, into which the alkaline matter is brought by drainage from 

 higher adjoining land, thus may have many times more of the soluble salts 

 in their soil than the average of the district ; and when these tracts become 

 diy and their moisture from a considerable depth is ch-awn upward and 

 evaporated at the surface it leaves a whitish-gray alkaline incrustation. 

 These low alkaline lands are rmfit for agriculture until they have been well 

 drained during several years, which may frecpiently be done by ditches 

 only a few feet deep, leading into lower watercourses. Excepting such 

 depressions and the marshes and sloughs before described, all the land of 

 this district has at least the very slight slopes necessary for free drainage, 

 and is well adapted for the cultivation of wheat, oats, and other cereals, 

 and of the common garden vegetables and small fruits that are suited to 

 the climate. The proportion of alkalme matter in the till soils wherever 

 they have natural di-ainage is not prejudicial but rather advantageous for 

 wheat and other grains. 



Along the axial belt of the Red River Valley plain alluvial clayey 

 silt usually borders the river to a distance of 5 to 10 miles on each side, 

 and other tracts are covered by fine lacusti-ine sediments of nearly the 

 same character, bordering the prominent delta plateaus. These areas have 

 a somewhat porous soil, composed mostly of very fine sand or rock flour 

 rather than true clay, being thus similar to the bowlder-clay or till; but 

 they have a somewhat less proportion of the soluble alkaline salts, which 

 in the process of aqueous deposition of these beds were partially removed 

 and can-ied away by the rivers into the sea. 



The deltas of sand and gravel, mainly modified drift washed away 

 from the melting ice-sheet and amassed in the margin of Lake Agassiz by 

 its tributary glacial rivers, also contain less alkaline matter than the till. 



