170 THE GLACIAL LAKE AGASSIZ. 



ablv in their central part the Fort Pierre shale, although no clear exposures 

 of it were found, so completely are the slopes and sununits mantled with 

 their very rough morainic accumulations and innumerable bowlders. On 

 the lake shore near the Fort Totten landing, and on the hills along the 

 next G miles eastward, a large projiortiou of the bowlders consists of Silu- 

 rian dolomitic limestone, like the strata which outcrop near Winnipeg in 

 Manitoba. Limestone masses 5 to 10 feet in diameter are occasionally 

 found in great numbers with the Archean granite and gneiss bowlders, 

 which elsewhere form on an average fully 99 per cent of the large bowl- 

 ders of North Dakota, so far as I have observed, excepting only in the 

 vicinitv of Butte Mashue (p. 15.")), where again these limestone bowlders 

 are very aliundant. 



Perhaps the most striking feature of this morainic belt south of Devils 

 Lake is its overwashed gravel and sand, which generally border the southern 

 side of the hills, descending from them in graceful slopes. This deposit is 

 most gi-andly exhibited between 3 and 4 miles east of the fort^ The upper 

 edge of the overwashed slope, there consisting of gravel and sand, with 

 rounded and subangular cobbles' of all sizes up to a foot in diameter, rests, 

 at about 225 feet above the lake, upon the southern side of the morainic 

 hills, with their- vast accumulations of bowlders, which rise only from 5 or 

 10 to 40 feet higher. The gravel and sand form a flat tract that declines 

 from the moraine at the rate of 30 or 40 feet per mile ; and these fluvial 

 beds have a considerable thickness, as is shown by Avatercom'ses which 

 have become channeled to depths of 50 and even 100 feet without dis- 

 closing bowlders. 



Devils Lake and Stump Lake were found by my leveling in August, 

 1887, respectively, 1,432 and 1,417 feet above the sea; but both were 

 depressed about 2 feet lower by the drought of the following two years. 

 These lakes, haAang very ii-regular outlines, with numerous windings and 

 long bays or arms, probably lie in valleys of a preglacial river and its trib- 

 utaries, elsewhere filled with drift. Devils Lake attains a maximum depth 

 of 75 or 80 feet in the eastern portion of its broadest area, and the north- 

 east arm of Stump Lake is said to be in some places 100 feet deep. The 

 ancient watercourse appears to have flowed eastward directly to the Red 

 River Valley. Its obstruction by drift during the Glacial period, and the 



