294 LEWIS G. WESTGATE 



glacier occupying the valley of Clear Creek, four miles to the south. 

 Here are two series of moraines. An inner one of late date is repre- 

 sented by two great lateral embankments rising 1,000 feet above the 

 valley bottom of Clear Creek and extending to the Arkansas. These 

 laterals do not unite toward the valley, but the lower moraines of the 

 same age between them are connected with gravels which follow 

 down the gorge of the Arkansas at levels not far above the river. 

 Outside the younger moraines there are, both north and south, 

 extensive areas of older moraine, better developed even than about 

 Lake Creek, and associated with higher gravels which lie above and 

 outside of the Arkansas gorge, and indeed are continuous with the 

 higher gravels about Lake Creek. The same differences in expres- 

 sion, in weathering, and in dissection by streams hold here between 

 the earlier and later moraines and gravels as in the valley of Lake 

 Creek, and make less probable any serious mistake in the interpreta- 

 tion which has been given to the features of that basin. 



II. PLEISTOCENE HISTORY OF THE UPPER ARKANSAS 



i. The terraces of the earlier glacial period. — One of the most 

 striking geological features of the Arkansas Valley is the great develop- 

 ment of the high-level gravels. From Granite, for a distance of 10 

 miles to the north, they occur nearly continuously on the west side 

 of the river. Opposite Hayden, on the east side of the river, the 

 graded rock-plain which has been noted as occurring to the south, 

 becomes covered by an extensive alluvial fan similar in physical 

 composition to the gravel terraces west of the river, with its lower 

 edge forming a terrace of the same altitude as the terrace on the west, 

 but rising steadily from this height back toward the gulches which 

 head in the Park Range back of Leadville. The earlier glacial 

 period was a time of ice-erosion. The streams leading down from 

 the glaciers were overloaded, aggrading was the rule, and extensive 

 wash-plains were formed. In the Arkansas valley aggrading went 

 on until the stream was flowing many feet, in some cases many scores 

 of feet, above its earlier level. Wash-plains extended continuously 

 along the main valley, and vast alluvial fans extended down from the 

 glaciated mountain valleys on either side. About Leadville these 

 fans are largely intact today; but on the west they do not seem to 



