PHYSICAL FEATURES OF COLORADO VALLEY, t-jj 



these minute ramifications liave been cut oiF; and we find canons 

 opening on the faces of the clitFs, the waters of which run backward 

 as above described. 



Let us suppose that w^e have a line of cliffs with an escarpment 

 facing the south. The rain, falling on the escarpment and in the re- 

 gion south of the cliffs, would run toward the south or along the foot 

 of the cliffs until it reached some more important water-channel ; the 

 rain falling on the plateau, from the brink of the cliffs backward, 

 would run toward the north, and the waters falling on this upper re- 

 gion would excavate channels for themselves, and, under proper con- 

 ditions, canons would be cut. As the cliffs are undermined and this 

 line carried back into the plateau, the area with a southern drainage 

 would be increased, the area with a northern drainage correspond- 

 ingly diminished, and, when the process had continued for a sufficient 

 length of time, we would find the southern edge of the plateau carried 

 away by this undermining process, until all the heads of the streams 

 were cut off and until the line had reached the canons. 



Gradually, during the progi-ess of erosion, the excavation of the 

 bottom of the canons would cease, as the supply of water running 

 throuo-h them would be cut off, and such caiions would have to be 

 considered as comparatively ancient. Such facts are frequently ob- 

 served in this caiion and cliff country. 



From such considerations, it seems that we may safely conclude 

 that the cliff topography has prevailed in that region for a long time. 

 There are evidences also that there were canons here before the pres- 

 ent canons were carved. The facts in relation to this matter can be 

 better stated when we come to discuss the geology of the region, 



Mr. G. K. Gilbert, a geologist of Lieutenant Wheeler's corps, in 

 a paper communicated to the Philosophical Society of Washington, in 

 1873, deduced a similar conclusion from an independent series of facts 

 observed in Western Utah. The basin of Great Salt Lake, a portion 

 of what Fremont designated the " Great Basin," has now so dry a 

 climate that its waters gather in its lowest parts and evaporate, and 

 have no outlet to the sea. In a former period, however, there was 

 more rain, the valley was filled with water to its brim, and in place 

 of the Salt Lake Desert, there was a broad and deep fresh lake, dis- 

 charging its surplus into the Columbia River. The epoch of this lake 

 Mr. Gilbert finds reason to consider identical with the Glacial Epoch, 

 and it was of limited duration. Among its vestiges are deposits of 

 fossiliferous marl, which are conspicuously contrasted with the gravels 

 and sand that now slowly accumulate in the same region, borne by 

 the intermittent streams that descend from the mountains. Where 

 the beds are superposed, the marls testify to a moist climate and the 

 gravels to a climate so dry that the basin was never filled with water. 

 But above the marls are found only scattered and thin deposits of 

 o-ravel, while below them the gravel-beds are omnipresent and of 



