THE TERRACES 685 



westward-facing terraces marked by steep or even vertical walls from 10 

 to 150 feet in height. 



Because of the low dip of the beds to the east the gain in elevation in 

 climl)ing a terrace scarp is lost in going east with the dip across the ter- 

 race flat, and this makes possible the many miles of vertical scarps which 

 flank the mountain. The rock is a coarse conglomerate with some in- 

 tervening finer and softer beds, and much jointed. The ice has taken 

 advantage of these master and minor joints in its work, and has carried 

 this forward with the regularity of a great quarry. A nearly horizontal 

 and softer bed has furnished the floor and a vertical joint the face, and 

 the courses have been carried east on a grand scale and with wonderful 

 symmetry. 



Whether the deep corries were made by ice or by water, it is assumed 

 that the terraces were formed later than the corries by the plucking of 

 the continental ice after it had reached such thickness that it had estab- 

 lished its south-southeastward motion. The lack of these terraces in the 

 bottom of the corries is explained by their depth and by the assumption 

 that the ice and till filling the corries was overridden by the main ice, 

 and so the plucking action was not marked there. The fact that the 

 terraces do run across the bottom of the northwest corries is explained 

 by tlieir position where they received the strongest impact of the ice, and 

 so their ])orders were worn down and their bottoms scoured free from till 

 and somewhat eroded with the formation of low terraces. 



Tlie fact that the terraces are found only on the west side is strong 

 indication that they were not caused by normal weathering and recession 

 of escarpments, for the beds are almost horizontal and emerge on the 

 east side of the mountain, and, if ordinary weathering had caused the 

 recession of the escarpments on the west side, similar and more perfect 

 escarpments should appear on the east side (since the dip would favor 

 them tliere), instead of an almost vertical wall. A great fault, however, 

 runs at the east foot of the mountain, and it may be urged tluit tlie.down- 

 throw of that fault removed an eastern portion of the mountain around 

 which the terrace scarps were formerly continued. 



But the clearly observed throw of that fault is only 100 feet, and could 

 not explain the sheer face of the mountain 700 feet high. Moreover, 

 these faults occurred near the end of the Triassic, and thus at the begin- 

 ning rather than at the end of the long erosion period during whicli the 

 scarps were cut back according to the hypothesis of erosion by weathering. 



A considerable fault runs at the foot of the high blufl^s east of the 

 ])oints indicated by letters "//' "^ /' ^'^^^^ 'T ^^ figure 1, plate 30, and 

 may be in part responsible for the exceptional height of these bluffs. 



