Vol. 6] Baker: (Jenozoic History of the Mohave Desert. 



375 



humid regions, although the relative importance of different 

 processes of erosion vary. The general processes of alluvial fan 

 formation on the boundaries between areas of topographic uncon- 

 formity are also so well understood as to need no explanation 

 at this time. But it is the precise application of the processes 

 by which terraces and alluvial fans are formed to the particular 

 problem in mind, namely, the dissection of alluvial debris slopes 

 in arid and semi-arid — and for that matter, to a certain extent 

 in more humid regions — that has not yet been clearly made. 

 For this reason it is desirable to outline the processes and to 

 trace their genetic influences upon the final product. 



There is postulated in the beginning a region of topographic 

 unconformity. In the beginning of the youthful stage of a cycle 

 of erosion, this topographic unconformity can be produced either 

 by faulting, by folding, by warping, or by tilting of a previously 

 existing surface, neglecting, as not pertinent to the discussion, 

 displacements along the littoral zone between sea and land. 

 Topographic unconformity may also exist at the mature stage 

 of an erosion cycle when a relatively resistant terrane is in 

 juxtaposition with one not so resistant. 



The first work of the agencies of gradation is to bring topo- 

 graphic unconformity into conformity. Degradation of the high 

 areas is accompanied by the complementary aggradation of the 

 adjoining lower lying areas, and this is accomplished regardless 

 of base-level, local or regional. So alluvial fans are formed in 

 the early stages of a normal cycle as well as of an arid cycle. 

 The causes of deposition of fan material are checking of gradient, 

 distribution of the stream over a larger surface, and evaporation 

 and seepage of the water. The rock falling from the cliff lodges 

 at its bottom, or rolling down an incline comes to rest Avhen 

 friction overcomes its momentum. The velocity of running 

 water is lessened on an abrupt decrease of gradient, causing 

 the deposition of much of the larger material carried in suspen- 

 sion. The deposition of debris successively carries the point 

 of initial checking of the velocity farther and farther up the 

 slope and would, if the process went on unchecked, finally mantle 

 the slopes to their very heads. But the destruction of this ideal 

 slope goes on apace by the agency of running water and subor- 



