. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 238.—Section of Coal-Field of Pennsylvania (after Lesley). the anticlinals have been broken or loosened by tension in bending; while the synclinals have been hardened by lateral pressure—and there- fore the anticlinals have yielded more easily to erosion. But Prof. Davis has shown * that such a supposition is at least not necessary. For example : if we have a series of undulating strata, some hard and some soft (Fig. 239), the erosion will be most rapid on the anti-. Fig. 239.—Ideal Diagram showing how,
. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 238.—Section of Coal-Field of Pennsylvania (after Lesley). the anticlinals have been broken or loosened by tension in bending; while the synclinals have been hardened by lateral pressure—and there- fore the anticlinals have yielded more easily to erosion. But Prof. Davis has shown * that such a supposition is at least not necessary. For example : if we have a series of undulating strata, some hard and some soft (Fig. 239), the erosion will be most rapid on the anti-. Fig. 239.—Ideal Diagram showing how, according to Davis, Synclinal Ridges are formed: full lines, actual surfaces and structure; dotted lines, original surfaces and structure; broken lines, former erosion-surfaces. clines and the hard stratum (a) will be reached and cut through first there. As soon as the soft stratum beneath is reached the erosion will be still more rapid, and valleys will be formed. This will be understood by careful inspection of the figure. 3. Strongly-folded or Highly-inclined Outcropping Strata.—These give rise to sharp ridges and valleys, the ridges being determined by Fig. 240.—Ideal Section across an Eroded Fold, consisting of Alternating Soft and Hard Strata (after NSe and De Margerie). the outcrop of a hard stratum. Fig. 240 is an ideal diagram, showing how such ridges are formed by erosion. In the Eocky Mountains, where they are finely shown on the flanks of the mountains, they are called " ; Fig. 241 represents this form of sculpture as it often appears. It is seen that every ridge is formed by outcrop of a hard sandstone, which has resisted erosion more than the intervening strata. Beautiful examples of this form are seen in parts of the Appalachian. Science, vol. xii, p. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may n
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