THE TWIN LAKES GLACIATED AREA 299 



contain only an insignificant amount of quartzite. Quartzite is 

 abundant in the Park Range about Leadville, but is not found on 

 either side of the valley about Lake and Clear Creeks. This dif- 

 ference in rock composition of the terrace gravels indicates that the 

 upper gravels about Lake Creek were almost wholly composed of 

 material swept in from the adjacent valleys, while during interglacial 

 time the Arkansas was sweeping that material away and bringing 

 large amounts of material down from the upper parts of the valley. 

 Incidentally it means that in early interglacial time continuous gravel 

 trains, above the level of the rock in the gorge, extended up towards 

 the head of the valley. 



3. Second glacial and postglacial. — The later glacial period 

 marked a new advance of the ice, the accumulation of the younger 

 moraines, and the development of a valley train in the rock-gorge 

 of the Arkansas, but was not marked by any change in the course 

 of the river. The later gravels occur in scattered terrace fragments 

 north to Granite Gulch. North of that point they are not distinct. 

 Postglacial time has been marked by the resumption of clearing by 

 the stream, and the removal in large part of the terrace made during 

 the later glacial advance. 



III. GLACIAL EROSION 



i. Valley form. — The cross-profile of a maturely glaciated moun- 

 tain valley will be typically as shown in Fig. 4; a broad U-shaped 

 valley below, surmounted by gentler, but still steep, slopes (AB) 

 made by undercutting, which, in a 

 region not too thoroughly glaciated, 

 will pass aboye into the gentle slopes 

 (AC) of preglacial origin. The most 

 striking feature of a view along FlG - 4-— Cross-profile of a glad- 



such a valley will be its broad a e va e ?- 



U -trough. This form is well shown both in the main valley of 

 Lake Creek and in those of its tributary valleys which were occu- 

 pied by ice. The best illustration is afforded by the middle and upper 

 part of Willis Gulch. No hanging valleys above, and no rock-sills 

 or buttresses below, mar the simplicity of its general form. It is a 

 remarkably regular example of the type. The middle part of several 



