38 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



however, we are not concerned. The fault scarp which forms the immediate eastern 

 boundary of the Otero Basin must hmit our investigations. This escarpment shows 

 evidence of two great periods of faulting and one minor period. Inasmuch as the strata 

 of the uppermost parts of the plateau are Permian, the earUest faulting may have taken 

 place in the Triassic or Jurassic eras. Cretaceous strata, however, he on the back slope 

 of the plateau. Hence the main faulting can not have taken place until late in that era 

 or in the Tertiary. After its occurrence the uplifted region was worn to a condition of matu- 

 rity, after which it was again raised by pronounced faulting at some time in the Tertiary 

 era. This movement, however, by no means brought the plateau to its present level. 

 This was not accomplished until the original fault scarp had been dissected into deep 

 valleys, its top had been battered back 2 or 3 miles from its original position, and its foot 

 had been concealed under a deep apron of piedmont gravels. Then renewed faulting 

 occurred and the piedmont gravels were cut in two, probably along nearly the same line 

 where the original faulting had taken place. To-day the old piedmont gravels of the 

 uphfted block can be seen as a much dissected terrace lying at an altitude of about 1,000 

 feet above the edge of the present plain. Below the terrace the topography is highly 

 rugged and youthful; above it, for a space, the topography is still somewhat rugged and 

 may be considered in the early stages of matm-ity ; while still higher, upon the main pla- 

 teau, as we have seen in an earher chapter, it is thoroughly mature. The date of this last 

 main faulting must be somewhere in the late Tertiary, but the process is probably not yet 

 complete. At the very base of the escarpment, from Alamogordo on the south to La Luz, 

 5 miles to the north, I traced a little fault scarp of very recent origin, and fm'ther investi- 

 gation would probably show that it extends much farther. The movement at the time 

 of the last faulting was in the same direction as during the major faultings of earlier 

 times, that is, the eastern side was uplifted. The piedmont deposits at the base of the 

 mountains were cut in two, and the eastern part now stands from 10 to 20 feet higher 

 than the western. Taken as a whole, the phenomena of the eastern side of the Otero Basin 

 are surprisingly like those of the eastern side of the basin of old Lake Bonneville, at the 

 base of the Wasatch Mountains near Salt Lake and Ogden. In other respects, also, the 

 two basins show marked similarity. 



On the west side of the Otero Basin the San Andreas Mountains (and also apparently 

 the Organ Range, farther to the south) assume the form of sharply tilted fault blocks, with 

 a precipitous escarpment forming the front slope and facing toward the east. The tops 

 of highly inclined Pennsylvanian limestones form the back slope which descends to the 

 Journada del Muerto east of the Rio Grande. Here the tilting of the block has been so 

 great that all semblance of a plateau or of an earlier topography has been destroyed. 



The floor of the Otero Basin has been deeply covered with deposits just as has the 

 floor of practically every basin in arid regions. The total depth of the filling is unknown, 

 but a railroad well at Alamogordo, near the edge of the basin, was sunk to a depth of 1,004 

 feet without reaching the bottom of the irregular succession of thin beds of fine gravel, 

 sand, and clay which compose the greater part of the basin deposits. These deposits 

 now form a large plain, very flat in the center and rising gently toward the edges, where 

 the materials change from saline deposits, silt, and clay to gravel. The width of the 

 basin from east to west is about 40 miles in the widest part; the length is over 100 miles. 

 On the south the plain rises gently to a flat divide and then falls away once more toward 

 El Paso and the Rio Grande. On the north it rises more rapidly after the main level 

 portion has been left behind, and ultimately it merges with the great plateau of central 

 New Mexico. The main line of drainage from the plateau to the basin floor is occupied 

 by a very recent lava flow which has sometimes been supposed to date back no farther 

 than the beginning of historic times. It begins in the vicinity of Carizozo and continues 

 southward nearly 60 miles. 



