28 THE GLACIAL LAKE AGASSIZ. 



Large tracts of the deltas tornied l^y the Sand Hill, Sheyenne, and 

 Assiniboine rivers have been heaped up by the wind in dunes or diittiug 

 sand hills, which vary in height from 25 to 100 feet. Their extremely 

 uneven contour, and their singular aspect, being partly covered by small 

 trees and bushes, but in many places wholly destitute of vegetation where 

 they are now gullied and drifted b}' the wind, make these hills a unique 

 element in the topography of the Red River basin. The worthlessness of 

 the dunes for agriculture is also in marked contrast with the great fertility 

 of the surrounding prairie, but they frequently include patches of good 

 pasturage in the intervening hollows. 



On the delta of the Sand Hill River, dunes 25 to 75 feet high have 

 been formed in irregular groups and series, scattered over a tract about a 

 mile wide and extending 3 or 4 miles sovith from the Sand Hill Rivei", 

 besides a single isolated group on its north side. Their highest points are 

 1,180 to 1,200 feet above the sea. 



Portions of the originally flat sand and gravel beds brought into Lake 

 Agassiz by the Shevenne have been blown into dunes, which vary from a 

 few feet to more than a hundred feet in height, and cover areas 5 to 15 

 miles long and 1 to 3 miles wide. Their summits are 1,100 to 1,150 feet 

 above the sea. The most southeastern of these large areas of conspicuous 

 sand hills of the Sheyenne delta lies close south of the Wild Rice River, 

 and is continued southeastward several miles by a lower lielt of such 

 hillocks to a high isolated cluster of them called the "Lightning's Nest" 

 (PI. VH). 



In Manitoba, wind-blown sand hills border the Assiniboine River in 

 many places along a distance of 60 miles, from near Brandon to near 

 Portage la Prairie, lying on the very extensive Assiniboine delta. 



The time of formation of the dunes on all these deltas was probably 

 soon after the withdrawal of Lake Agassiz, before vegetation had spread 

 over the surface. The winds could then erode more rapidly than now, and 

 heaped up these hills of sand in nearly their present size and height; but 

 it is evident also that their forms have been constantly undergoing slight 

 changes since that time. 



