THE TWIN LAKES GLACIATED AREA 295 



have been so extensively developed, and are now found in distinct, 

 but not conspicuous, remnants on the shoulders of Mount Massive 

 between the glaciated valleys. 



Emmons 1 and Hayden 2 have suggested that the gravels are lake 

 deposits of an interglacial period. They do not seem to be inter- 

 glacial deposits, as they are closely related to the earlier moraines; 

 and interglacial time was an epoch of valley-cutting and not of 

 valley-filling. But are the gravels lake deposits? To the writer 

 there is nothing to suggest that they are not stream-built gravel- 

 plains. The material is mainly coarse, bowlders up to one and two 

 feet in diameter being very abundant. It is slightly or not at all 

 stratified. Where coarse gravel and fine gravel alternate there is a 

 rude stratification, but in the coarse gravel stratification is wanting. 

 If the material had been deposited in a body of standing water, a 

 marked development of foreset beds (delta structure) should be 

 found throughout the gravels; but any such structure is lacking. 

 At one point only was any considerable amount of fine material noted, 

 just north of Cache Creek (Fig. 1, 5), at the base of Lost Canyon 

 Mountain. Here there is a considerable body of fine gravel, and 

 some clay, in the plain. This is explained by the protected position, 

 between the points of discharge of Lake Creek and Clear Creek. It 

 is probable that the more rapid building up of the plain both to the 

 north and south may have left depressions which might be filled with 

 finer material or even clay, but such deposits of fine material are very 

 local. 



Not only the character of the gravels, but the disposition of the 

 surface, indicates stream-work. Emmons notes that south of Lead- 

 ville the gravels occur on the spurs back from the river at an eleva- 

 tion 1,000 feet greater than along the river, and infers the elevation 

 of the Park Range over the valley by that amount. But a similar 

 rise of the gravels occurs west of the Arkansas, though the gravels 

 do not reach the altitudes they do near Leadville. Gravel fragments 

 occur on the foothills of Mount Elbert several hundred feet above 

 the level of the terrace along the river. These gravel terraces rising 



1 Emmons, Geology and Mining Industry 0} Leadville, Second Annual Report, 

 U. S. Geological Survey, pp. 220, 229. 



2 Annual Report for 1873, p. 53, and for 1874, p. 52. 



