136 MARGARET BRADLEY FULLER 



waters had cut the new eastern outlet below the level of Muggins 

 Gulch after the ice had left the eastern (North Fork) valley. 



Proofs of the exceedingly youthful character of unit 3 are derived 

 from a study of the gradient of the present Thompson Valley. 



The gradient of the Big Thompson River averages from Estes 

 to the foothills about 100 feet per mile. About 3 miles west of 

 Drake along unit 3 the gradient is nearly 250 feet per mile for a 

 distance of over 2 miles. This is the steepest fall along the river's 

 course which is here between nearly vertical canyon walls cut alike 

 in granite and schist. As far as erosion by the stream alone is 

 concerned this stretch is the youngest topographic feature along 

 the entire course of the river, -This portion of the valley is solely 

 the result of stream erosion established by early glacial drainage 

 across a low divide which developed into a steep and winding canyon 

 widening east to Drake and west to Estes Valley. 



The early Pleistocene glaciers left units 2 and 4 much as they 

 are today except that recent stream erosion resulting from an 

 upwarp of at least 100 feet over much of the area has cut a canyon 

 50 feet to 100 feet deep in the floors of units 2 and 4 and has cleaned 

 out the early glacial debris from the botton of the valley. 



In late Pleistocene (probably late Wisconsin) times ice again 

 advanced over part of the course of the Big Thompson but to a 

 much less extent than in early Pleistocene times. The limits of 

 this invasion mark the boundaries of unit i where massive terminal 

 moraines separate the areas which have been influenced by both 

 early and late glaciation (unit i) from that which was covered by 

 the early ice only (unit 2), 



During and perhaps previous to late Wisconsin glaciation the 

 Big Thompson Valley stood about 100 feet lower than at present. 

 At any rate, debris from the melting of the ice was deposited as a 

 thick layer of sand and gravel over the flatter parts of the valley 

 floor outside of the late Wisconsin ice area. 



As the Wisconsin ice vanished an upwarp of at least 100 feet 

 affected most of the area. The river promptly dissected most of 

 the terrace deposits and began sinking its channel into the bedrock, 

 producing a canyon within a canyon in units 3 and 5 and letting a 

 narrow canyon nearly 100 feet deep into the broad floors of units 2 



