UPPER ARKANSAS VALLEY. 347 



lakes. In other places coarse gravel continues right up to the bluff mark- 

 ing the former margin of the Arkansas glacier, proving that the waters that 

 were poured from above into these open spaces found ready exit into the 

 subglacial waterways of the main glacier, and in such cases the alluvial 

 mesa was formed subaerially, ending in a steep bluff because piled against 

 the side of the Arkansas glacier. The symmetry of the U-shaped valleys 

 bordered by bluffs of glacial sediments or moraines is better preserved in 

 case of the shorter glaciers. The enormous amount of morainal matter 

 brought down by the longer lateral glaciers formed dams that obstructed 

 their flow and forced them to wander in search of an outlet. On the great 

 moraines of the Lake Creek glacier, which are situated northeast of Twin 

 Lakes, and which formed in part the lateral moraine of the Willow Gulch 

 glacier of Mount Elbert, we find remarkably sudden transitions between 

 morainal ridges and glacial sediments. The region has been ^Jrospected by 

 placer miners, and thus were revealed the following facts: At the top of the 

 great moraine a shaft was dug 98 feet in gritty clay. The digging is on a 

 small level place. Two hundred feet west a steep ridge rises perhaps 50 

 feet above this flat and is composed of bowlders and other coarse moraine 

 stuff. In several places are mounds or small mesas that rise 50 to 100 

 feet above the rest of the moraine, which are proved by tunnels and shafts 

 to be composed of clay and fine sand. These local masses of fine sedi- 

 ments in the midst of moraines were probably deposited in small glacial 

 lakes like those of the marginal region of the Malespina glacier, described 

 by Russell. The retreat of the Mount Elbert glaciers here described seems 

 to have been quite rapid until we reach the base of the mountain. Here 

 the principal tributary, the Willow Gulch glacier, formed several frontal 

 alluvial terraces at different elevations. Going up the mountain from this 

 place we find only a few small deposits of glacial gravel, the streams of 

 the shortening glacier becoming too feeble to transport much sediment. 

 But the shrinking of the glacier is marked by a series of retreatal moraines 

 that are found every half mile or so up to timber line. Near the base of 

 the mountain the morainal matter is nearly all well glaciated, and often 

 contains so much rock flour, clay, and fine ddbris as to resemble the till of 

 New England. Going up the mountain we find the glaciation becoming 

 less intense, till at the last we find rock piles in the characteristic form of 

 moraines with few signs of attrition. This delineation is only intended to 



