642 THE GLACIAL LAKE AGASSIZ. 



are shales of the Fort Pierre aud Laramie formations, so soft and easily eroded 

 wherever exposed to weathering that glacial marks are not preserved. The sand- 

 stone of Willow Kiver, however, outcrops also in the same district on Turkey and Ox 

 creeks, but does not there retain striated surfaces. Farther west, apparently this 

 stratum of sandstone occurs in the bluft's of the Souris River, near its most southern 

 bend, and in hills within the area of the loop formed by this river, where other glacial 

 striae may probably be found. 



