THE TWIN LAKES GLACIATED AREA 309 



feet above the valley bottom, the rock along the side of the gulch has 

 slipped down and out in a series of lobes whose festooned and 

 ridged surfaces express the flow within the mass of the landslide. 

 In a number of instances landslides have dammed up the streams 

 and caused small lakes; indeed, the small lakes in the gulches are 

 almost uniformly formed in this way, and lakes occupying rock 

 basins are rare. 



MOUNTAIN FORM 



Mountain form is an expression of two factors: rock structure, 

 including original form and internal arrangement, and erosive 

 processes. So far as the first of the two factors is < oncerned, conditions 

 in the Twin Lakes region are simple. The rocks are all completely 

 crystalline, and are granite, granite porphyries, and granitic gneisses. 

 Their composition is such that no great difference may be expected 

 in resistance to weathering, and their surface distribution bears this 

 out, for they appear to be largely independent of topography. The 

 porphyritic granite has already been referred to as one of the rocks 

 most characteristic of the valley of Lake Creek, and it is possible 

 that its somewhat greater capacity for weathering may have deter- 

 mined the course of the preglacial Lake Creek; but the ridges which 

 separate the valleys tributary to Lake Creek are in places porphyritic 

 granite, in places gneiss, and the difference in the rock seems to make 

 but little difference in the character of the divides. In some instances 

 the jointing of the rock plays a part in shaping minor details of form, 

 the rough serrate ridges about La Plata being worked out on a 

 jointed compact gneiss which holds its form well and gives the pecul- 

 iarly jagged outlines to the lower divides; but petrographically 

 the Twin Lakes area is a granitic complex, and differences in form 

 are to be explained by differences in the character of the erosive 

 processes. 



The erosive processes which have been at work in the area are 

 two, stream-erosion and glacial erosion. The larger features of the 

 region were worked out by streams in preglacial time. These larger 

 features were profoundly modified in places by ice-erosion in Pleisto- 

 cene time. 



The preglacial topography of the Arkansas valley and of the adja- 

 cent Park and Sawatch ranges can only be inferred from those 



