i893. 



PHYSICAL GEOLOGY IN COLORADO. 207 



The surface drainage is in the direction of this slope, and then along 

 the foot of the cliff to one of the permanent rivers. The directions of 

 these permanent rivers bear no such relation to the cliffs, and give 

 rise to an important observation. 



Powell has pointed out that when the Green River in its south- 

 ward course encounters the great range of the Uinta Mountains, 

 lying directly across its path, instead of being diverted, it cuts right 

 through the range and follows its own course. Again, in crossing 

 the Yampa Plateau, the same river cuts directly through the highest 

 ground in the district in a deep gorge appropriately named Split 

 Mountain Cafion. This entire independence of the mountain and 

 plateau systems characterises all the main water-courses of the 

 region. Several rivers rising on the eastern slopes of the Wasatch 

 run directly westward through the heart of the range, and fall into 

 Great Salt Lake. Of such phenomena there is only one explana- 

 tion ; the rivers existed before the mountains and plateaux ; in other 

 words, we have a striking example of an "antecedent" drainage 

 system. As an elevation was gradually formed in the path of a river, 

 the latter exercised its most active erosive power upon the part of 

 its bed thus raised, and was able to keep its channel cut down to the 

 proper level, and maintain its right of way. In order that the river 

 should not be diverted, erosion must have been able to keep pace 

 with elevation throughout the whole time, and we have here a clear 

 proof of the very gradual operation of the elevatory forces. 



Such movements of elevation as those just indicated have 

 affected in varying degree the whole region, though sometimes the 

 amount of the elevation can be shown to have differed by thousands 

 of feet in adjacent tracts. The rivers, meanwhile, have maintained 

 approximately their original position relative to sea-level. Hence 

 arises one of the most remarkable features of this part of America : 

 the rivers flow in deep narrow gorges or canons instead of meander- 

 ing in broad valleys like the streams of a country comparatively 

 stationary. The cafion-type of valley, which is by no means con- 

 fined to arid regions or to rocks of any particular character, seems to 

 belong especially to regions undergoing uplift, where the rivers, 

 entirely occupied in cutting downward to maintain their grade, have 

 had no energy to spare for eroding laterally. 



Bearing this principle in mind, the well-preserved canon-form 

 of the valleys in the Colorado basin shows that the process of eleva- 

 tion initiated in middle Tertiary times is still in progress. A very 

 striking evidence of this is seen on the abrupt western flank of the 

 Wasatch Mountains. The generalised structure of the range is that 

 of a sharp uplift bounded to the west by a fault of at least 6,000 feet 

 throw. The movement along this fault has doubtless been effected 

 gradually, and a portion of it is clearly of recent date, for where the 

 gorges in the mountains debouch on the great plain of the Salt Lake 

 the fresh fault-scarp is often seen truncating the terminal moraines 



