130 MARGARET BRADLEY FULLER 



The river's course is a series of sharply twisted meanders. Where 

 straight stretches do occur they He along dipping beds of schist and 

 the more abrupt turns follow the prevailing joint systems in the 

 granite and schist. The river first crosses granite with small 

 amounts of schist, then crosses an area chiefly of granite from Estes 

 Valley to Loveland Heights. To the east the granite is more and 

 more interrupted by schist until at Drake, schist and massive peg- 

 matite dikes predominate. East of Drake, through the fourth 

 and fifth units, the course is parallel to the dip of the steeply dipping 

 beds of schist for most of the distance. Both units, although so 

 different in appearance, are developed in much the same sort of 

 rock. The schist, except locally, is the least resistant, the pegmatite 

 next, and the granite the most resistant of the rocks exposed along 

 the river's course. Hence canyons in schist at the lower end of the 

 valley and broad, open stretches in granite near the upper part must 

 owe their sequence to factors other than simple stream erosion. 



PRE-WISCONSIN GLACIATION 



It is believed that there are, within the area drained by the Big 

 Thompson, evidences of a much earlier Pleistocene glaciation than 

 the late Wisconsin invasion. There are four principal lines of 

 evidence of this earlier glaciation. 



I. U-shaped valleys. — ^The U-shaped outlines of portions of the 

 main valley have already been pointed out in the description of the 

 second and fourth units. Mountain valleys which have been occu- 

 pied by glaciers always show a characteristically U-shaped profile, 

 rounded and scoured by the ice to its upper limits on the valley 

 walls, with the surrounding unglaciated peaks left steep and 

 serrate. The profile of Mont Rose Valley (Figs, i and 2) shows a 

 typical U-shape, with Palisade Mountain exhibiting steep and sharp 

 peaks above the rounded outline. The Estes Valley is also dis- 

 tinctly U-shaped, but the ice evidently rose to overlap its margin 

 north and south of Estes Village. To the east, however, the 

 profile of Mounts Olympus and Pisgah exhibit sharp peaks which 

 stood well above the ice in the valleys below. 



In addition to these regions already described there are flat 

 areas of oval shape such as the Orchard and Cedar Park (Fig. 4). 



