CAUSE OF ALKALINE SOILS. 589 



the very fertile soil of this district and producing the niag-nificent harvests 

 of wheat which are its principal export and source of wealth. 



A still larger proportion of the drift upon the prairie district of Lake 

 Agassiz was supplied from the Fort Pierre, Niobrara, and Fort Benton 

 shale formations of the Cretaceous series. They are mostly soft shales, 

 however, and therefore have supplied no bowlders; nor are they usually 

 represented conspicuously by pebbles of the till and in gravel deposits, 

 excepting near the western border of the ancient lake, where the Pembina 

 Mountain escarpment and plateau, the basal part of the Tiger Hills, and 

 the Riding and Duck mountains, consist of these shales. Many streams 

 flowing down from these highlands have cut deep ravines and vallcA's in 

 their frontal escarpment, and have spread much shale gravel outwanl for 

 several miles along the watercourses on the Red River Valley plain. The 

 till or glacial drift on this western margin of the lacustrine area and on 

 all the plateau country extending thence westward has also a consider- 

 able ingredient of shale gravel. But the greater part of the material 

 contributed from these sliale beds to the glacial drift is mingled with the 

 pulverized Archean granite and gneiss and Paleozoic limestones, doubtless 

 generally far surpassing these as a constituent of the finely comminuted 

 rock flour which is the most abundant element of the bowlder-clay or till. 



The portion of the till thus received from the Cretaceous beds has 

 given to its soil the somewhat alkaline character which is perhaps the most 

 noteworthy quality distinguishing the soil of this distinct and of the plains 

 on the west, in contrast with the soil of the Northern States and Canadian 

 provinces east of Lake Agassiz. The sulphates of magnesia and soda, with 

 other soluble salts, together termed "alkali," which are present in consid- 

 erable amount in the glacial drift of the Red River Valley and the western 

 plains, are almost wholly due to the conti'ibution of the Cretaceous shales 

 to the drift. This soluble mineral matter was contained in the Cretaceous 

 ocean, and much of it became imprisoned and stored up in the very fine 

 clayey sediments of that time. On the areas of these shale formations 

 beyond the limits of the glacial drift the soil is far more alkaline than 

 within this region, where Archean, Paleozoic, and Cretaceous strata have 



