WESTERN AND SOUTHERN PARTS OF THE VALLEY. 9 



These foot slopes are mainly formed by a series of coalescing alluvial 

 fans composed of the coarser detrital material distributed by tor- 

 rential streams debouching from the mountain barriers. Since the 

 larger streams of greater carrying capacity emerge from the west- 

 side ranges, the fans are here much more extensive and have built 

 their way farther out into the valley, pushing the valley trough 

 toward its eastern margin. The waters of Saguache Creek appear 

 to have carried comparatively little of the coarser detrital material, 

 and the fan structure here is imperfectly developed. The soils of 

 the fan deposits occur as a wide margin of sloping, naturally well- 

 drained, porous, gravelly soils, bordering the flat area of deficient 

 drainage occurring in the lower parts of the valley basin. Much of 

 the material of the lower outlying portions of the fans has been 

 assorted and deposited in the waters of the lake formerly occupying 

 the valley basin and intermingled with the finer deposits of the valley 

 floor, or the older detrital and lacustrine fan deposits have been 

 covered by more recent gravels and other stream-distributed foot- 

 slope material. 



In the bottoms and flood plains of the larger or alluvium-carrying 

 streams extensive areas of recent stream-deposited sediments occur, 

 usually of darker color, heavier texture, and more compact structure 

 than the soils of the foot slopes. The deposited areas are of gently 

 sloping to level surface and are frequently subject to overflow, and 

 sometimes support tree growth, consisting principally of cotton- 

 wood and willow. Soils of this character are of wide distribution, 

 not only in stream bottoms, but over much of the lower valley plains, 

 where as superficial deposits they obscure the earlier lacustrine sedi- 

 ments. Along the line of contact between the foot-slope deposits 

 and the alluvial soils of the stream bottoms and valley floor the 

 soils have been modified by intermingling of recent stream-deposited 

 material. Extensive modification in mineral composition, texture, 

 and structure has been brought about by decomposition or weather- 

 ing of the mineral fragments subsequent to their deposition. In 

 certain localities the effect of winds has caused modification of the 

 original material by the formation of extensive areas of wind-drifted 

 sands. 



SOILS OF THE WESTERN AND SOUTHERN PAETS OF THE VALLEY. 



Although a large number of soils occur throughout the valley, 

 many of the types are of uniform and widespread occurrence. 

 This is true particularly of the more thickly settled and highly 

 developed farming districts of the western slopes covered by the 

 great alluvial fan of the Rio Grande. This fan, having its apex 

 at the entrance to the canyon of that stream near Del Norte, extends 

 east with gently decreasing slope into the valley and includes the 

 18595 Cir. 5212 2 



