TIME OF RECENT GLACIATION 259 



Pine River drainages. Special mention has been made of them in the 

 La Plata folio, where they are represented on the geologic map. 



TIME OF RECENT GLACIATION 



A noteworthy feature of all of the glaciated San Juan region is the 

 very slight erosion that has been accomplished since the disappearance of 

 the ice, shown not only in the channels of the streams, but also in the 

 slight modification in form to which the moraines and drift have been 

 subjected. More erosion, as one might expect, has been acomplished in 

 the soft Cretaceous shales of the foothills and plateau region than in the 

 massive volcanic or pre-Cambrian rocks of the interior. The moraines 

 show little or no effects of weathering, although they have been cut in 

 two by the streams flowing in the main valley. The cuts, however, are 

 sharp, and the exposures on either side are generally fresh and uncovered 

 by vegetation, while in the uneroded portion of the moraine the forms are 

 almost as distinct as they were at the time the moraine was first deposited. 

 This is also true of the lateral moraines, although from their more ex- 

 posed positions on the hillsides more or less of their material has been 

 carried downward by creep or wash and deposited at lower elevations. 

 The freshness of the materials constituting the moraines is very striking ) 

 the boulders are superficially almost entirely unoxidized and many show 

 distinct striations. This holds true, not only of the harder pre-Cambrian 

 rocks which constitute a large part of the Animas moraine, but also of 

 the Paleozoic and later rocks which are quite abundant, boulders or peb- 

 bles of compact limestone being not uncommon. 



Many of the higher cirques are practically in the condition in which 

 they were left by the ice, their floors being bare and no erosion having 

 been accomplished by the streams which now occupy them. The greatest 

 erosion has naturally taken place at points intermediate between the 

 cirques and the flood-plains of the main streams ; thus, in Needle creek, a. 

 tributary of Animas river, the stream has intrenched itself to a depth of 

 2 or 3 feet in the old rock floor, but at many places actually flows over 

 glacially polished surfaces. Even in regions of softer rocks the post- 

 glacial erosion has been very slight. 



As already mentioned, ice has been observed in one of the high cirques 

 of this portion of the Rocky mountains, and in other areas in Colorado 

 actual glaciers, although of small size, are known to occur, as, for example, 

 the now well known Arapahoe glacier in the front range, 20 miles west 

 of Boulder. All of this evidence is of interest and importance in show- 

 ing the relatively recent time in which actual glacial conditions existed 

 in the San Juan mountains and probably throughout the Rocky moun- 



