BEACH RIDGES. 199 



this steep line of hig-hlands, to river action din-ing the time of epeirogeuic 

 elevation which closed the Tertiary and introduced the Quaternary era, not 

 in any important degree to glaciation, and least of all to shore-cutting by 

 the glacial lake. 



Beaches. — The course of the shore of a large glacial lake is usually 

 marked by a smoothly rounded beach ridge of gravel and sand (see PI. 

 VI and fig. 6, p. 26) wherever the land is a slope of till sinking slowly 

 beneath the ancient water-level. Like the shore accumulations of present 

 lakes and of the seacoast, the glacial lake beaches vary considerably in 

 size, having in any distance of .5 miles some portions 5 or 10 feet higher 

 than others, due to the unequal power of waves and currents at these parts 

 of the shore. Moderate slopes bordering the greater glacial lakes were 

 favorable for the formation of beach ridges, and such ground frequently 

 displays many beaches at successive levels which marked pauses in the 

 gradual elevation of the land when it was relieved of its ice burden, and in 

 the subsidence of the lake as its outlet became eroded deeper or as the 

 glacial retreat uncovered new and lower avenues of discharge. 



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 corresi^onding 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 g-reater 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 several miles with remarkable uniformity of size, having* 

 a central thickness of 10 to 15 feet and a total width of 20 to 30 rods, are 

 found on areas of till that rise with a gentle slope of 10 or 1.5 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.^ 



' "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. I. 1890, Chapter II. 



