160 EFFECTS OF RIVERS ON THE EARTH'S SURFACE. 



From what has been said of the action of rivers, it is evident that 

 their effects upon the physical features of a country are varied and 

 interesting. The tendency of all descending streams of water is the 

 same, to equalise the surface of the earth, to remove its ridges and 

 asperities, and smooth its depressions and fissures. 



The degree in which they respectively perform this work depends, 

 first, on the amount of atmospheric and local influences in wasting the 

 surface of the higher ground, and bringing materials for the rivers to 

 act upon. Hence the rapid waste of high Alpine tracts exposed to 

 fluctuating heat and cold, to storms, avalanches, and glaciers. Hence 

 the streams of sand and pebbles, which are carried from the gritstone 

 hills of England ; and, on the contrary, the almost unvaried purity of 

 the springs which break from the Carboniferous limestone. 



The second circumstance which determines the modifying power of 

 the river is its own volume and velocity, and these are principally 

 dependent on the physical geography of the region. The datum of 

 the volume of water flowing in any valley is principally useful for 

 comparison with the observed effects; the kind of effect produced 

 being determined by the velocity of the current. 



If we conceive that in its first fury a river may have power enough 

 to sweep along even large blocks of stone, but that its velocity gra- 

 dually diminishes, there will be a certain point where these large 

 blocks will be left by the enfeebled current, pebbles will roll farther, 

 coarse sand will travel beyond, and the finer sediment will be moved 

 on till the languid waters permit their slow and equal deposition. 

 This gradation of deposits is always observed in examining valleys 

 of sufficient length and elevation. The deposits in the upper 

 parts are tumultuous and confused, in the lower regions level and 

 regular. 



A third circumstance, of still more importance than the others, 

 serves to regulate the action of the river. This is the form and 

 character of the valley itself. However produced, there can be no 

 question that the present aspect of almost every valley in the. world 

 is smoother and more equalised than it was formerly, since we see 

 evidently, and take as a principle, that the characteristic effect of 

 denuding agents in action is to reduce continually the inequality 

 which remains. We may, therefore, easily, for each valley, restore in 

 imagination its ancient condition, remove the sediment from its 

 expanded meadows, and leave, instead of level or gently sloping 

 plains that wind smoothly round the hills, and ascend far up 

 toward the sources of the stream, deep chasms between cliffs rent 

 asunder by convulsion, or eroded by tidal attrition, or the solvent 

 chemical action of carbonated waters. That such has been the origin 

 of many valleys is evident. That many of these may have been 

 partly cleared out, and others wholly excavated by violent floods, 

 sweeping over and denuding the land during its elevation from 

 the sea, may be considered as proved. But we may content ourselves 

 for the present with the datum that the fundamental features of 

 valleys are not always the result of the excavating action of their 



