270 



STKUCTURE COMMON TO ALL KOCKS. 



Standing on the top of Warm Springs Mountain, Virginia, ten or twelve 

 parallel ridges may be counted, each with long slopes on one side and 

 steep slopes on the other, like, billows ready to break. The crest of 

 each ridge is determined by the outcrop of a hard sandstone. Such 

 ridges may be formed either by the outcrop of successive sandstones, 



Fig. 241. 



<F7i S 



-Parallel Ridges. 



as in Fig. 241, or else by the successive outcrop of the same sandstone, 



as in Fig. 242. 



In ridges formed in this way the relative angle of slope on the two 

 sides of the ridges will depend on the dip of the strata. If the strata 



Fig. 242.— Parallel Ridges in Folded Strata. 



be vertical, the two slopes will be equal. If the strata are inclined, the 

 longer slope will be on the side toward which the strata dip ; and the 

 difference of the two slopes will increase as the angle of dip becomes 

 less. This is shown in Fig. 243 (a, l, and c). Finally, one slope may 



Fig. 243. 



coincide with the face of the hard stratum, as in Fig. 240. This case, 

 therefore, passes by insensible gradations into the next, viz. : 



4. Gently -inclined, almost Level Strata. — These, by erosion, per- 

 haps under peculiar climatic conditions, give rise to a succession of 

 broad, nearly level tables, coincident with the face of a hard stratum, 

 terminated each by a vertical or nearly vertical cliff. This form of 

 sculpture is developed on a magnificent scale in the Colorado plateau 

 region. Fig. 244 is a bird's-eye view of three such tables, each 20 to 60 

 miles wide, and terminated by cliffs 1,500 to 2,000 feet high. Fig. 245 

 is an ideal section of such tables and cliffs — the dotted lines showing the 



