PHYSIOGRAPHIC DEVELOPMENT IN COLORADO 137 



and 4. At the present time these inner canyons are being rapidly- 

 degraded, and at no points outside of unit i is there any aggradation 

 taking place. 



SUMMARY 



The composite course of the Big Thompson River Valley com- 

 prises five units linked together as a result of the complex physi- 

 ographic history of the area from the Continental Divide to the 

 foothills district. 



Lines of evidence point to two distinct river systems flowing 

 independently but in the same general direction to the foothills. 

 Canyon-cutting by these two streams was interrupted by early 

 Pleistocene glaciers which filled in the headwaters and crept far 

 down each valley toward the foothills. The glaciers blocked the 

 normal drainage outlets of the western (Muggins Gulch) river 

 system and its waters traveled east over a low divide to the eastern 

 (North Fork) valley which, because of its lower altitude was freed 

 from ice while glaciers still filled the western area. The drainage 

 lines thus established from the western to the eastern river systems 

 became the connecting link in the new river valley — the Big Thomp- 

 son of today. Late Pleistocene glaciers affected only the head- 

 waters of this new system and are in no way responsible for the 

 general course of the Big Thompson. 



Thus the present river valley is made up of the glaciated head- 

 waters of one river system joined by a young precipitous stream- 

 carved canyon to the glaciated middle portions of another river 

 system and the whole extends east through a river-cut gorge across 

 the foothills and the plains of eastern Colorado to join the North 

 Platte River east of Greeley. 



