RELATION OF BEACHES TO DELTAS. 247 



Waves driven toward the shore by storms gathered the beach gravel and 

 sand from the deposit of till or other drift which was the lake bed ; and cor- 

 responding deposits of stratified clay, derived from the same erosion of the 

 till, sank in the deeper part of the lake. But these sediments were evidently 

 of small amount and are not commonly noticeable on the sheet of till which 

 forms the greater part of the lacustrine areas. Where the beaches cross 

 delta deposits, especially the fine silt and clay that lie in front of the delta 

 gravel and sand, they are indistinctly developed or fail entirely. On the 

 other hand, the most massive and typical beach ridges, often continuous sev- 

 eral miles with remarkable uniformity of size, are found on areas of till that 

 rise with a gentle slope of ten or fifteen feet per mile. Under the influence 

 of irregular contours of the shore, however, the beach deposits assume the 

 form of bars, spits, hooks, loops, and terraces, of which Gilbert has given a 

 careful classification, with analysis of the interactions of waves and currents 

 by which they were made.^"^ 



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

 to strong winds is required for the formation of sufliciently large and power- 

 ful 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 undulating 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 formation, but often made more con- 

 spicuous by erosion of the outer portion of its original area by waves and 

 shore currents when the lake fell to lower levels. 



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

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

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

 many of the resulting sand dunes, 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 accumulations, being sand derived 



*"The Topographic Features of Lake Shores:" Fifth Annual Report of the U. S. Geol. Survey, 

 1885, pp. 75-123 ; " Lake Bonneville : " Monographs of the U. S. Geol. Survey, vol. 1, 1890, chapter 11. 



