42 INITIAL CAUSES. 



in large degree to fluvial action as well. During the glacial period erosion of 

 the hardest rocks or of softer materials to great depths was the universal 

 accompaniment of glacial movement. In the Rocky Mountains and Sierra 

 Nevada the extreme conditions which rock invaders must meet are often the 

 direct outcome of glacial scouring in the past. 



The action of wind-driven ice on exposed shores is a striking feature of many- 

 lakes in Minnesota and Wisconsin as well as elsewhere. Shores otherwise 

 similar are differentiated by the grinding and pushing action of the ice. Bare 

 shores are modified in various ways, while those covered with vegetation are 

 denuded more or less completely (plates 4 a, 5 c). 



DEPOSIT. 



Significance. — ^Deposit is such a regular and often such an immediate con- 

 sequence of erosion that it is desirable to emphasize the fact that this essential 

 relation, which is so fundamental to physiography, is of little or no consequence 

 in the development of vegetation. Material eroded in one part of a drainage 

 basin must in the usual course be deposited in another part, and in both cases 

 it bears a direct relation to the development of the river system. This would 

 be in no wise true of the development of the vegetation in the two areas, 

 especially if the latter were in different climatic regions. Even in the same 

 climatic region it is true only of final or subfinal stages. This latter fact, 

 however, indicates no essential relationship, since all initial causes in the region 

 give rise to seres which reach the same climax. It must also be recalled that 

 the great deposits of marl, peat, travertine, sinter, and volcanic dust bear no 

 relation to a preceding erosion. 



The relation of deposit to the future development of vegetation depends 

 upon a number of factors. These are: (1) the agency of transport; (2) kind 

 of material; (3) manner of deposition; (4) rate, depth, and extent; (5) place 

 and distance of deposit. These determine the rate at which the sere can 

 develop, the physical conditions which the invaders must meet, the climax 

 vegetation from which they can be drawn, and the effect of migration. 



Agents of deposit. — If the term is used in the inclusive sense, the agents of 

 deposit are: (1) running water; (2) ground-water; (3) wind; (4) glaciers, ice, 

 and snow; (5) volcanoes; (6) gravity. Plants and animals also build deposits, 

 but these are naturally considered under biotic agencies and under reactions. 

 Just as it is practically impossible to draw a line between the loosening of 

 material and its transport, so it is often equally impossible to separate trans- 

 port from sedimentation. In any area of deposition the two are going on 

 simultaneously, the dropping of part of the load carried by water, for example, 

 permitting the further transport of the remainder. Deltas and alluvial fans 

 are especially fine examples of the sorting due to the interaction of these two 

 processes. They make it clear that any unit deposit is due to the varying 

 distances of transport of the particles, as well as to the fact of their fall. How- 

 ever, in the case of a single particle, it is evident that this is first transported 

 and then deposited, after which it may be transported and deposited again 

 and again. In the study of a sediment actually forming, the last phase of trans- 

 port must be included in deposition. 



As is true of erosion also, two agents may interact in effecting deposition. 

 The ordinary relation between two agents is successive, as in the case of 



