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action of the subaerial forces to a greater extent in the parts imme- 
diately adjacent to their courses than they do at a greater distance. 
Moreover, the seaward transportal of the degraded material helps 
the rivers in many ways to erode their own beds. ‘Thus, on the 
whole, the general tendency of rivers is to cut vertically, and to 
deepen the valleys they occupy. Where the effects of weathering 
upon the rocks are at a minimum, the rivers have it all their own 
way, and the square-cut channel resulting from their unaided 
action is left with hardly any modification. Magnificent examples 
are presented by the caifions of Colorado. In the few cases where 
the rate of vertical erosion effected by the river and the lateral 
erosion of the rocks under the action of the weather are equal, 
banks are left with slopes of 45°.* More commonly, the rate of 
weathering near a river exceeds the rate of vertical erosion, and a 
valley with sides enclosing a very wide angle is the result. But in 
every case the nature of the rocks affected is apt to produce 
important modifications of detail. 
We will now pass from general considerations to review some 
of the minor details bearing upon the history of the rivers specially 
under notice. 
If we follow up nearly any stream flowing into the Eden, care- 
fully noting the variation in the form of its valley from point to 
point, we shall find, in nearly every case, that although the actual 
size of the valley may be very small where the stream flows through 
the New Red, yet when we reach its higher parts where its course 
traverses older and harder rocks, the valley expands into one of 
much-more striking proportions. The valley of Dale Beck, which 
extends upwards from Nunnery Walks to Croglin Fell by way of 
Renwick, is a good case in point. We find that in the lowlands 
of Edenside, where its course lies through the New Red, the valley 
is a shallow channel, whose depression below the general level of 
the adjoining surface is hardly perceptible from a little distance. 
But where the same stream flows over the rocks that form the 
Escarpment, it widens out into one of the largest valleys along the 
whole fell side. The explanation of this apparent anomaly is that 
* Upon this point see a paper by the writer in Geol, Mag. II. Vol. 2, p. 325. 
