THE UNA WEEP CANON. 7 8 5 



the margin of the plateau have been very much disturbed, which would 

 easily have obliterated all traces of the original caiion. Entering the 

 body of the plateau, the cafion suddenly assumes a greater breadth. 

 As it increases in depth, it exposes a series of several hundred feet of 

 stratified rocks, below which the granite comes to the surface. At 

 the crest of the plateau the lower two thirds of the walls are of gran- 

 ite, the upper 1,000 feet being sedimentaries. From its northeastern 

 end the caiion gradually widens from a bed-breadth of a hundred 

 yards to one of at least a mile ; at the crest, the canon suddenly nar- 

 rows to a mere stream-way, with rugged, vertical walls. 



It is purely a caiion of erosion. There is no sign whatever of any 

 local disturbance which could account for its existence. The strata on 

 the two sides are perfectly conformable. 



To attribute this caiion to the streams now occupying it is mani- 

 festly absurd. Not only are they utterly insignificant in comparison 

 with the amount of erosion which has taken place, but no similar 

 streams, of any magnitude whatever, could have cut the canon down 

 at the divide ; neither could the western one have cut back into the 

 plateau to any such extent. There is no lateral slope toward this 

 caiion to determine the drainage of any considerable area of the 

 plateau in this direction. It is plainly the scene of the defeat of a 

 large stream, in its struggle to maintain its ancient course of a victory 

 of volcanic over aqueous forces. Another thing is equally apparent 

 that the stream here diverted came from the eastward and not from 

 the westward, and that it was the Grand River. This is shown by 

 the following considerations : (1) The general slope of the country, 

 disregarding local accidents of topography, is from the northeast 

 toward the southwest ; (2) the direction of the course of Grand River 

 above this caiion, which is almost precisely in line with (3) the char- 

 acter of its course, which shows that it antedated all other uplifts, and, 

 as the Uncompahgre Plateau, from its trend and association, must have 

 been coexistent with the rest, it must have antedated this also ; (4) 

 the features of the caiion itself afford the strongest possible evidence 

 of a stream flowing southwest through it. The profile, with the sum- 

 mit east of the crest of the plateau, the slow descent west of the 

 summit to the crest of the plateau, and the rapid descent beyond the 

 crest, point unmistakably to this conclusion. The plan of the canon 

 is no less clear in its indications. It is well known that a rapid stream 

 erodes its bed downward ; a sluggish one, on the other hand, erodes 

 laterally, thus broadening its bed. Here we have precisely these phe- 

 nomena. Beyond the crest of the plateau, where the slope is great, 

 the caiion is very narrow ; while east of the crest, where the velocity 

 of the stream must have been very much lessened, it widens rapidly, 

 and then gradually diminishes in width. 



At what stage in the rise of the Uncompahgre Plateau the river 



abandoned the unequal contest and took its present course around the 

 vol. xx. 50 



