PHYSIOGRAPHIC DEVELOPMENT IN COLORADO 135 



of the Thompson. When the ice finally melted from the Estes 

 basin to the west, drainage had been permanently established along 

 its present lines throughout most of the Thompson Valley. 



The combined features of U-shaped valleys, remnants of till, 

 glacial potholes, and interrupted and reorganized drainage furnish 

 ample proof that glaciers invaded, eroded, and melted from portions 

 of the Thompson Valley in early Pleistocene times. 



HISTORY OF THE UNITS IN THE BIG THOMPSON VALLEY 



The five different units comprising the Thompson River Valley 

 are believed to have been the result of several processes affecting 

 two distinct river systems. 



The essential elements of these processes in chronological order 

 comprise: (i) normal stream erosion; (2) Early Pleistocene valley 

 glaciers; (3) Early glacial drainage; (4) late Pleistocene valley 

 glaciers. 



Normal stream erosion first determined the courses of both river 

 systems. Probably both streams developed steep-walled canyons 

 along much of their courses. The areas occupied by units i and 

 2 of the present Thompson River Valley were the upper reaches of 

 the western (Muggins Gulch) river system, while units 2 and 4 com- 

 prised the middle portions of the eastern (North Fork) river valley. 

 Unit 3 is the connecting link between these areas and is the youngest 

 topographic feature of the present Thompson Valley. Unit 5, 

 which was the lower part of the eastern river valley, is probably 

 the only unmodified portion of these early systems which were 

 incorporated into the Thompson. As far as the present analysis 

 goes, this gorge (unit 5) has been a simple stream-carved canyon 

 throughout its history. 



The stream cycle of erosion was interrupted by early Pleistocene 

 valley glaciers which filled in the headwaters and middle reaches 

 of both river systems modifying the outlines of the parts which 

 later became units i, 2, and 4 from V-shaped gorges to open U- 

 shaped valleys. Drainage from the western ice-filled valley escaped 

 both through Muggins Gulch and over the low col to the east 

 through the eastern stream valley to the foothills. The later outlet 

 became the site of all the drainage from the west when the 



