THE TWIN LAKES GLACIATED AREA 297 



earlier course. Here it quickly sunk its channel through the gravels 

 into the underlying granite, and its interglacial valley is in consequence 

 narrow and gorge-like. North of its rock-valley, toward Leadville, 

 the Arkansas does not flow in rock, and its interglacial valley is much 

 wider, averaging three-quarters of a mile, though not less deep than 

 to the south. This difference in valley- width is, of course, due to the 

 difference in hardness of the materials in which the valley has been 

 opened. There is probably a double reason for the failure of the 

 river to lay bare rock to the north. North of Lake Creek glaciers 

 occurred on both sides of the valley, there was not the unsymmetrical 

 development of wash that farther south forced the river to hug the 

 east side of the valley, and it therefore kept more nearly its preglacial 

 course. Further, the preglacial Arkansas was flowing in a graded 

 rock valley which probably had a gentle gradient, while the inter- 

 glacial gravels were laid down by rapid streams handling coarse mate- 

 rial and building that same material into steeper slopes. Later 

 erosion would naturally reach rock more generally along the lower 

 course of the river where the gravels were less thick. 



As the Arkansas cut its valley during interglacial time, the side 

 streams were lowering theirs. Those from the east were flowing 

 over a rock-graded plain thinly veneered with river gravel, and as 

 they cut down, they dissected the plain and largely removed the 

 gravel, so that now it occurs only in scattered bowlders over the 

 divides between the tributary streams. The slope of these tributaries 

 from the east steepens toward the main stream; they have not quite 

 been able to keep up with the main stream in downward cutting. 

 The small stream which enters the Arkansas through drift from the 

 west at Hayden has swept away the whole terrace for a half-mile in 

 width along its lower course, and at Lake Creek the terrace has been 

 quite cleared away west of the river. No permanent streams enter 

 the Arkansas between these two, but wet-weather streams originating 

 on the old moraine and terrace have cut out wide, shallow valleys, 

 whose depth is determined by the ledge of hard rock which the streams 

 have found at the edge of the river. North of Lake Creek this action 

 has cut out several broad, transverse valleys, but has not destroyed 

 the original surface of the higher terrace. Between Lake Creek 

 and Clear Creek there are different conditions. Here several streams 



