Mountains of the South 225 



Buckman Springs; and the larger area of McCain's Plateau, 

 lying between La Posta and Jacumba valleys. These and other 

 elevated old age surfaces are largely mantled with deep residual 

 soil and rotten rock. Certain of the minor blocks rose a good 

 deal less than the immediately adjacent ones, and they appear 

 like relatively sunken fault blocks. Good examples are Viejas, 

 Cottonwood, Morena, and Potrera valleys, the valley just east 

 of Jamul Mountain, and the valleys just northwest of Tecate 

 Mountain. 



The Plateau Made up of Fault Blocks 



The fault blocks, which constitute the highland or plateau 

 region, are generally stepped-down from the east at successively 

 lower levels toward the west and southwest. The triangular, 

 quadrangular, and pentagonal shapes of the mountain masses 

 and associated valleys strongly indicate that they are fault 

 blocks rather than results of stream erosion. Tracts that are 

 comparatively flat, some of them surrounded by steep mountain 

 walls, cover many square miles within the highland area. The 

 floors of these valleys are comparatively smooth and slope gently 

 toward the streams to which they are tributary, in distinct con- 

 trast to their rugged surroundings. Each basin is crossed by 

 one or more streams, but there is nothing to indicate that the 

 basins were formed by erosion of the streams. The highland 

 basins are grouped into three belts trending parallel with the 

 axis of the main range. The correspondence in the elevations 

 of the basins in each of the three belts, and the fact that the 

 belts are successively higher from the coastal region eastward, 

 indicate that the high flat areas so widely distributed through- 

 out the region are remnants of an extensive rolling plain that 

 sloped gently toward the west in an earlier geologic time, and 

 the highland basins are down-faulted segments of the ancient 

 plain. The nearly flat summit areas of the plateaus that inter- 

 vene between the highland basins and valleys are interpreted as 

 old-age surfaces recently elevated by faulting. Previous to the 



