200 THE GLACIAL LAKE AGASSIZ. 



Deltas. — A broad expanse of water exposed along a distance of many 

 miles to strong winds is required for the formation of sufficiently large and 

 powerful waves to erode cliffs or accumulate well-defined beach ridges; but 

 the area of any glacial lake, small or large, may be partly occupied by 

 deltas brought into its margin by tributary streams. These deposits at the 

 mouths of small brooks are often only a few rods wide, while the deltas of 

 rivers, especially those supplied with much englacial drift from the melting 

 ice-sheet, sometimes extend many miles in a flat or moderately xmdulating 

 plain of gravel and sand, lying at. the level which the surface of the lake 

 held during the accumulation of the delta, or within a few feet above or 

 below that level. But at the mouth of the river forming the delta it was 

 frequently built up in a fan-shaped mass to a considerable height, the head 

 of the alluvial slope being in some instances 50 feet or more above the 

 lake. The delta plain is generally bounded on its lakeward side by a 

 somewhat steep descent, partly due to the ordinary conditions of delta 

 foi-mation, but often made more conspicuous by erosion of the outer por- 

 tion of its original area by Avaves and shore currents when the lake fell to 

 lower levels. 



Winds in many places have channeled and heaped the surface of the 

 more extensive deltas, acting most efficiently as soon as they became un- 

 covered from the lake and before they could be overspread by vegetation; 

 and many of the resulting sand dunes (see PI. VII, p. 28), which frequently 

 range from 25 to 100 feet in height, though mainly covered by grass, 

 bushes, and trees, are still undergoing slight changes of their form by wind 

 erosion. All the dunes on the areas of the glacial lakes Agassiz, Dakota, 

 Souris, and Saskatchewan, occur on delta deposits; but the great tracts of 

 dunes about the south end of Lake Michigan belong wholly to beach accu- 

 mulations, being sand derived from erosion of the eastern and western shores 

 of the lake, whence it has been borne southward b}^ shore currents, espe- 

 cially during northern gales. None of the beaches of our glacial lakes are 

 large enough to make dunes like those on Lake Michigan, though the size 

 and depth of Lake Agassiz, its great extent from south to north, and the 

 character of its shores, seem equally favorable for their accunnilation. It 



