194 THE GLACIAL LAKE AGASSIZ. 



clay. A long interlacustriue epoch is known by overlying alluvial gravel 

 and sand. The second rise of the lake reached the level of overflow 

 apparently after the water surface had been long held within 5 to 20 feet 

 below that level, forming a widely spread deposit of white marl and the 

 well-defined highest beach ridges and eroded cliffs, which Gilbert names 

 the Bonneville shore-line. The time required for the great amount of wave 

 work at this level would be made possible by long-continued undergrbund 

 drainage from the lake through the alluvial deposit of Cache Valley, over 

 which a slightly higher rise of the lake finally gained a superficial outflow 

 to the Columbia River, and then rapidly cut a channel 375 feet deep in the 

 alluvium to a sill of limestone. At this lower level, marked by the Provo 

 shore-line and deltas, the lake was held for a long* time, peAaps occasion- 

 ally interrupted by dry climate and fall, of the water too low to maintain 

 its outlet. 



Glaciers descending the canyons on the w'est front of the Wasatch 

 Range attained their maximum extent, pushing their moraines into Lake 

 Bonneville, during the time of formation of the Provo shore-line. From 

 these moraines, and from those of the Sierra Nevada extending into the 

 Pleistocene area of Lake Mono, the glaciation of the Cordilleran region is 

 known to have been contemporaneous with the epochs of humid climate 

 and extension of lakes in the Great Basin, the interlacustriue epoch being 

 attended probably with a nearly or quite complete departure of the glaciers 

 and ice-fields on the mountains. 



LAKE AGASSIZ XND OTHER GLACIAL LAKES.^ 



A glacial lake, according to my use of the term in this volume and 

 elsewhere, is a body of water bounded in part l)y a barrier of land ice. 

 The lake may be hemmed in by a glacier, as the Merjelen See, or by a 

 continental ice-sheet, as Lake Agassiz. And the same name is also appli- 

 cable to the lakelets, wholly bounded by ice, Avhich are occasionally 

 formed, attaining a considerable depth and extent and appearing in the 

 same places during the summers of successive years, on the surface of 



' The following descriptions and discussion of this class of Pleistocene lakes were originally pre- 

 sented in a jiaper before the Geological Society of America ("Glacial Lakes in Canada," Bulletin, 

 G. S. A., Vol. II, pp. 243-276). 



