GRAVEL AS A RESISTANT ROCK 497 



combined with the effect of gravity and the rapid mechanical 

 erosion, would doubtless cause more rapid removal of gravel 

 than of granite on account of the dominance of the factor of bodily 

 transportation. 



PIEDMONT GRAVELS NEAR SILVER CITY, NEW MEXICO 



Near the town of Silver City, N.M., best shown between there 

 and the smaller town of Central, seven miles directly to the east 

 (see U.S.G.S., Silver City quadrangle), there lies a gravel deposit 

 of Piedmont nature which presents points of particular interest 

 in connection with the thesis just presented. The road from Silver 

 City to Central follows closely along the inner or mountainward 

 margin of this deposit (Fig. 1) which extends from here southward 

 for 40 or 50 miles as a part of the gravel fan of a great interior basin 

 which, with its tributary basins, covers a large area in the south- 

 western part of the state. 



The gravel plateau, as it will be called, in the portion under 

 consideration between Silver City and Central, has a slope to the 

 south of about 100 ft. per mile and strikes approximately east and 

 west. It is characterized by great uniformity and evenness as 

 seen from any point on the plateau surface. One sees merely a 

 monotonous plain of gravel, horizontal as one looks to the east or 

 west, but sloping always toward the south. This appearance of 

 great evenness applies, however, only to the remnants as seen from 

 a point on the plateau surface, for, particularly near the northern 

 or mountainward margin, it is considerably dissected by streams 

 which, flowing outward from their sources in the mountains, have 

 carved long, usually nearly straight and parallel valleys down the 

 dip of the plateau (Fig. 2). These valleys are cut to a depth 

 of about 150 ft. at the maximum. As one follows them out toward 

 the desert plain they gradually become shallower and finally dis- 

 appear altogether. Degradation there gives place to aggradation. 



The gravel of the plateau is composed of rocks found in place in 

 the mountains and the whole character and relationship of the 

 deposit points clearly to its origin as a Piedmont accumulation of 

 gravel spread out from the adjacent mountains to the north at some 

 earlier time before dissection set in. 



