156 GEOLOGY OF SOME OF THE RIVER-SCENERY OF DERBYSHIRE. 



them flowing for miles, perhaps, through broad and open valleys, 

 and then through comparatively narrow and steep-sided gorges. 

 It will be invariably found that where the valley is broad and open 

 the river is flowing through soft and easily denuded strata, and 

 that where it is narrow and steep-sided, the rocks are hard and 

 unyielding. 



In the Colorado River of the west, which empties itself into the 

 Gulf of California, we have the rare instance of a river which has 

 been allowed to carry on the process of cutting its "trench" 

 without the interference of any other denuding agency. This river 

 flows for over 200 miles through a rainless district. There has, 

 therefore, been no rain or other atmospheric agency, to round off 

 the edges of the trench, to wear away its walls, and to scoop out a 

 valley. We find in consequence that the river flows through a 

 narrow gorge, the walls of which are quite vertical, and vary in 

 height from 3,000 to 6,000 feet ; in other words, in many 

 parts the " trench " is more than a mile deep. At various 

 points between the river and the top of the trench may be seen, 

 on protruding pieces of rock, small isolated patches of river-gravel, 

 similar to the gravel in the bed of the river, marking former levels 

 of the stream. A river with banks quite vertical and a mile high 

 is of course only possible in an area free from atmospheric 

 agencies. Here an enormous thickness of solid rock, flanking 

 the stream on each side, has escaped denudation owing to there 

 being no rain or other denuding force to supplement the action 

 of the river. 



In Derbyshire we frequently find rivers behaving in what at 

 first sight seems to be a most eccentric manner. They appear 

 to have intentionally gone out of their way to discover and 

 encounter difficulties, and to have deliberately chosen to cut 

 through hard rocks, when it was open to them to find an easy 

 channel through soft strata. Take for instance the course of the 

 Derwent near Matlock. This river flows from Rowsley in a 

 broad open valley of Yoredale shales. About a mile from 

 Matlock Bath it leaves these soft shales and has cut a deep and 

 comparatively narrow gorge into the carboniferous limestone, 



