EROSION. 39 



If we consider wind erosion and deposit instead of that by water, it seems 

 to afford the clue to the puzzle. While wind erosion is of much less impor- 

 tance, it still plays a large part, as seen in the hundreds of thousands of square 

 miles covered by dunes, sand-hills, and loess deposits. Here the process is 

 totally opposed to base-leveling, as the sand or dust is blown from strand or 

 plains into dunes or hills. The significant fact is that the hills and crests are 

 driest, the hollows wettest. Controlled by water-content extremes, seres of 

 totally different intitial stages arise in these two areas, converge more and 

 more as they develop, and terminate in the same climax. In consequence the 

 actual explanation appears to lie in the fact that in the usual erosion by water, 

 soil and water move together. The water which falls on a hill leaves the crest 

 or slope with the soil it has eroded away. When it reaches the ravine, stream, 

 or lowland it deposits its load, only to be itself entrapped in large degree. 

 Thus, it is evident that topography, with soU texture, is the great middleman 

 distributing rainfall to the various habitats as water-content. It is this 

 relation which one finds repeated again and again in a drainage basin, in youth, 

 in maturity, and in old age, wherever erosion and deposition occur. The age 

 of the basin seems to affect the relation only in so far as it determines the nima- 

 ber or steepness of slopes on which erosion can occur, or the area of lowland 

 where deposits can accumulate. 



EROSION. 



Nature. — ^The removal of soU or rock by the wearing away of the land 

 surface is erosion. In the case of rock it is often preceded by weathering, 

 but the process consists essentially of corrosion, the picking up of the loose or 

 loosened material and of its transportation. Weathering is too universal 

 and too well understood to warrant discussion here. In so far as plants play 

 a part in it, it will be considered under "Reactions" (p. 83). A distinction 

 between corrasion and transport is difficult if not impossible. With wind and 

 water, the picking up of weathered particles involves carrying them as well, 

 while gravity transports or affects transportation without picking the material 

 up. In the beds of streams or glaciers, however, corrasion plays an essential 

 part in freeing material for transport. Where some part of the rock is dis- 

 solved in the water, in the process of corrosion, the distinction from transport is 

 also very slight. 



As a rule the agent which picks up the material is the one which transports 

 it, as is evident in the erosion of a gully or the scooping out of a sand-hill or 

 dune. Often, however, material freed by gravity, as in talus slopes, is trans- 

 ported by water or wind. The distance of transport varies within the widest 

 limits. In residuary soils the conversion of the rock takes place by weathering 

 alone. Loosened material may be carried a few millimeters into the cracks 

 of rocks, or it may be carried hundreds of miles and into totally different habi- 

 tats and regions. The distance of transport naturally determines the place of 

 deposit, but it will suffice to consider the latter alone. 



Agents of erosion. — ^The great agents of erosion are water and wind at the 

 present time. The action of ice, while of paramount importance during the 

 glacial period, especially in transport, is now limited and local. The effect 

 of gravity, combined with weathering, is less extensive than that of wind and 

 water, but the areas so produced are of great service in studying succession, 



