494 JOHN LYON RICH 



the attack of frost or for the entrance of percolating water, while 

 the comparatively small size of the units diminishes the activity of 

 insolation as a disintegrating agent. For these reasons, while a 

 massive quartzite, for instance, may be as resistant as gravel to 

 disintegration due to either mechanical abrasion or chemical decom- 

 position, it will be more likely, especially if in larger masses, to 

 suffer more from the effects of insolation and frost. 



From the foregoing it is clear that stream gravels, particularly 

 the coarser ones, may properly be looked upon as concentrates of 

 the most resistant elements of the rocks from which they are derived. 

 It follows that a gravel of such a nature will be more resistant to the 

 agencies of disintegration than the original rocks. From its very 

 nature and origin a gravel deposit should be expected to offer great 

 resistance to the normal agencies of sub-aerial denudation. This 

 resistant quality is particularly significant in the development of 

 the topography of gravel deposits, since, disintegration being at a 

 minimum, bodily removal of the component units of the gravel is 

 necessary for their reduction ; and, as we shall point out later, bodily 

 removal, too, is at a minimum except along the immediate courses 

 of good-sized streams. In the latter situations this may be readily 

 accomplished, but away from the actual stream course the removal 

 of the material must necessarily be very slow. The importance 

 of this point in relation to the dissection of the alluvial fans along 

 the base of a mountain range will be more fully elaborated on a 

 subsequent page. 



The second characteristic of gravels which makes them resistant 

 to the disintegrating and erosive forces which would wear them 

 down is their porosity and the consequent comparatively slight 

 development of surface drainage on the gravel areas. A gravel 

 deposit of moderate coarseness offers the maximum of favorable 

 conditions for the absorption and storage of the rain which falls 

 upon its surface. This hinders the formation of small surface 

 streams, and since, as we have seen, disintegration is at a minimum, 

 and the removal of the gravel is almost entirely dependent upon the 

 transporting action of such streams, the gravels are doubly pro- 

 tected from removal. 



From the foregoing theoretical considerations we should expect 



