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University of California Publications in Geology [Vol. 9 



The contour of the subaerial front for the greater part of the 

 time of its recession is not in reality a straight, or even a gently 

 sinuous, line, but is actually indentate. At the indentations are canons 

 or gullies and from these emerge the greater part of the detritus 

 which forms the embankment and which is distributed radially from 

 an apex in. or at the mouth of, every canon or gully. It results 

 from these conditions: (1) that the longitudinal profile and contour 

 of the general embankment are sinuous; (2) that the sinuosity is 

 most acute near the mountain and flattens out toward the valley; 

 (3) that the mean limit of the coarse detritus is in plan a sharply 

 sinuous or serrate line and the mean limit of the medium detritus 

 a smoothly sinuous line; and (4) that in the depressions of the longi- 

 tudinal profile the angle of slope of the embankment is less than in 

 the median lines of the indvidual fans, and the amount of water 

 available for transportation is less. Since the conditions referred to 

 persist throughout the greater part of the time of the accumulations 

 of the embankment, the internal structure of the latter is regularly 

 more complex than might be inferred from the discussion of the ideal 

 case. This complexity will manifest itself stratigraphically chiefly 

 in the alternate thickening and thinning of the fanglomerate and the 

 arkose in the elongation of these formations parallel to the range, 

 and in an appreciable longitudinal component of dip of the actual 

 depositional stratification in opposite directions on the two sides of 

 the axes of the individual fans. In the later stages of the general 

 process, when the subaerial front is relatively small, this complexity 

 will become less marked ; but if the front be reduced to nothing in 

 one part of a range while in neighboring parts it is still a prominent 

 feature, the variation in thickness and in dip of the deposits of the 

 embankment will be again accentuated. 



The Subaerial Front. — In the discussion of the ideal case the sub- 

 aerial front is represented by implication as a mountain slope having 

 a straight contour. This is a conceivable case on the assumption of 

 perfect homogeneity of material and structure; but as a matter of 

 fact none of the mountains of the desert are perfectly homogeneous, 

 and all their subaerial fronts have a more or less indentate contour. 

 The indentation may be slight in the early stages of front recession, 

 is most pronounced in the middle stages and becomes less intricate 

 in the later stages. By supplying a larger surface to atmospheric 

 attack the development of gullies and canons accelerates the general 

 process of recession, and in so far as the acceleration is local, it 



