GRAVEL AS A RESISTANT ROCK 505 



At first, with steep, exposed slopes, disintegration, through frost 

 and insolation, and erosion will be rapid. The streams, while 

 powerful enough to carry the loosened material down the steep 

 slopes, will be unable to transport it across the lowland below. 

 Piedmont fans of coarse gravel will accumulate along the mountain 

 base. As time goes on the fans will continue to grow at the expense 

 of the mountains. During this stage the fans are the seat of con- 

 tinual deposition, the mountains of continual waste and removal. 

 Finally there must come a time when the mountains have become 

 so lowered that the streams are no longer flowing over steep slopes. 

 As this stage is approached, disintegration and decomposition 

 within the mountain area will become relatively more important 

 and the rocks will be reduced to a finer condition before being car- 

 ried off. The streams will no longer be overburdened with sediment 

 too coarse to be carried beyond the base of the mountain. At 

 this point the upbuilding of the fans at the immediate mountain 

 base must cease while the locus of deposition is shifted farther out 

 because the stream load, being of a finer nature, may be carried to 

 a greater distance before deposition occurs. 



This is the turning-point in the history of the mountain range. 

 From now on, both mountains and fans will be subject to denuda- 

 tion or degradation. If both the fans and the mountains were 

 worn down at an equal rate, the whole area would merely lose in 

 elevation without any marked change in the relations of mountains 

 and gravels. Since, however, according to our thesis, the gravels 

 will suffer from erosion less than the rocks of the mountains, 

 differential erosion becomes an important factor. As the slopes 

 decrease and decomposition plays an increasingly important role 

 while the material furnished to the streams becomes finer and less 

 in amount, the burden of the streams becomes less and they are 

 able to cut where deposition was in progress before, and will sink 

 their channels into the Piedmont fans. 



Since the mountains are lowered faster than the gravels, a low- 

 land will gradually develop, beginning first near the position of the 

 inner margin of the gravel at the time of the change from aggrada- 

 tion to degradation. If the base level of the streams is sufficiently 

 low, this lowland may eventually come to include the whole of the 



