566 PROCEEDINGS OF THE ALBUQUERQUE MEETING 



the development of cirques. Wlieu due to the latter cause the opposiug slopes 

 of tlie divide becorue precipitous aud sharp arretes are formed. 



The liaugiug valleys iu Big Timber cauyou aud elsewhere indicate a consid- 

 erable amount of glacial erosion, at least locally, but the relatively narrow 

 gateway iu Sweetgrass canyon does uot show much modification by ice action. 

 The rocks both within and without the contact zone of the sediments with the 

 eruptives are weaker than those of that zone, so that the canyons broaden 

 both above and below that region. The contact zone would thus mark a belt 

 of minimum deepening, which at the time of occupation by glaciers was prob- 

 ably the site of ice falls, as it now is of cascades. Above this zone the max- 

 imum deepening, as indicated by the hanging valley in Big Timber, may have 

 been as much as 300 or even 500 feet. Below this zone the maximum deepen- 

 ing may have equalled or even exceeded that figure ; the streams have not yet 

 penetrated the glacial filling. But the additional depth of 300 or 500 feet 

 produced by glacial action iu a cauyou where the sides rise 2,000 or 3,000 feet 

 above the valley floor, cannot be regarded as a very extensive modification of 

 pre-Glacial form. 



The topographic features of the Crazy mountains thus seem to be mainly 

 pre-Glacial. The ice remained long enough to deepen aud modify the form of 

 existing valleys and to develop the cirques aud arretes so common in the 

 higher mountains. 



Post-glacial Erosiort 



At the heads of the canyons the streams are cutting new gorges for them- 

 selves beneath the old trough floors. These are still so immature that the 

 rims of the hanging valleys are barely notched by them as they leap in cas- 

 cades to the valley floors below. The glacial lakelets are for the most part 

 unfilled and undrained. In the north branches of the east fork of Cottonwood 

 (Pine) creek, sharply cut V-shaped gorges were observed in the fillings of the 

 glacial valleys, but these gorges had not penetrated the filling, which was 100 

 feet or more in thickness, and had not reached grade. In the hard rocks of 

 the contact zone, cascades have developed aud some incision has been accom- 

 plished, while iu glacial fillings below this "fall line" the streams have 

 reached grade, without cutting through the filling, and rush along through 

 stony flood plains. In the lower canyons of some of the larger creeks, such as 

 Cottonwood (Pine) and Sweetgrass, the streams have cut into the bed rock 

 and developed gorges 100 feet or more in depth. In Sweetgrass creek a beau- 

 tiful meander belt has been established, for a distance of about 3 miles, iu the 

 weak rocks near the lower end of the canyon. 



Within the area affected by the glaciers post-Glacial erosion has accom- 

 plished little in the way of readjustment of drainage. Indeed, the drainage 

 may be said to be highly immature. Outside the glaciated area, however, ero- 

 sion has progressed .steadily from the glacial time to the present. The larger 

 streams have developed broad flood plains, such as those of the Shields river 

 and the Yellowstone, and uiauj' of the smaller ones have I'eached grade. 



Summary 



During the Glacial period the Crazy mountains were the seat of local glacia- 

 tion. Cliff, valley, and piedmont glaciers were formed, the largest being on 



