GRAVEL AS A RESISTANT ROCK 493 



RESISTANT QUALITIES OF GRAVEL 



It is natural to look upon gravels as weak rocks which may easily 

 be removed by the agencies of denudation. While this is doubt- 

 less true for sand or possibly even for fine gravel, it is a view which 

 does not hold true of gravel of a coarser nature such as accumulates 

 at the base of mountain ranges either in arid or humid climates, or 

 of river gravels of the coarser type such as the Lafayette gravels 

 of the Mississippi Valley. 



There are two essential reasons for the resistant quality of gravel 

 as regards denudation. These are: (1) the selected nature of the 

 material; (2) its porosity. As regards the first of these, gravel is a 

 composite rock made up of units, each of which is selected on the 

 basis of its ability to withstand the action of the agencies of de- 

 struction to which all rocks are subjected. These agencies of dis- 

 integration are both mechanical and chemical. With respect to 

 the mechanical, gravel may be looked upon as a residue which has 

 survived the rolling, pounding, and abrasion incident to its trans- 

 portation along the stream course, an experience which, if the jour- 

 ney be a long one, effectively grinds down and destroys all but the 

 most resistant of the materials subjected to it. 



From the standpoint, also, of rock decomposition gravel is par- 

 ticularly resistant, for it is a rock whose component materials 

 are severally selected on the basis of their ability to withstand such 

 decomposition. In a region of normal development where there 

 has been no interference with normal conditions by such accidents 

 as glaciation, the stream gravels represent, in the main, only rocks 

 or fragments of rocks which, by virtue of their resistant qualities, 

 have been able to survive unchanged the decomposition and 

 mechanical disintegration which has effectively destroyed the rock 

 surrounding them. They have undergone successfully the ordeal 

 which has destroyed the neighboring rock. They are therefore 

 still able to resist further subjection to the action of the same agen- 

 cies of change. 



Physically also, gravel is especially fitted to resist disintegration 

 because in it the component fragments are reduced to compact 

 units unbroken, as a rule, by fractures or other lines of weakness. 

 The surfaces are generally smoothed and give little opportunity for 



