OCR Text |
Show 172 EXPLOHATION OF TIIE CANONS OF TDE COLORADO. little faster than the degradation of its surface, but, as it comes up, the wearing away is extended still farther out on the flanks, and the same beds are attacked in the new land which have already been caiTied away nearer th9 center of the fold. In this way the action of erosion is continued on the same bed from the up-turned axis toward the down-turned axis, and it may and does often happen that any particular bed may be entirely carried away, with many underlying rocks, near the former line, before it is attacked near the latter. Now, as the beds are of heterogeneous structures, some hard and others soft, the harder beds withstand the action of the storms, while the softer beds are rapidly canied away. rrhe manner in which these beds are degraded is very different. 'l'he softer are washed from tho top, but the harder are little affected by tho direct action of the waters-they are torn down by another process. As the softer beds disappear, the harder are undermined, ~nd are constantly breaking down; are crushed, more or less, by tho fall, and scattered over, and mingled with the softer beds, and are carried away with them. But the progress of this undermining and digging down of the cliif is parallel with the upturned axis of the fold, so that the cliffs face such an axis. When the fold is abrupt, so that the rocks on either side are made to incline at a great angle, ridges are formed, and this topographic structure of a country may be found even in a land of rains, though the ridges will usuall~ be low, rounded, and more or less irregular, while in a dry climate they w1ll be steep and regular, and will usually culminate above in a sharp edge; but where the rocks are slightly inclined, terraces will be formed, with well defined escarpments. It is interesting to note the manner in which the textures of these hard capping rocks affect the contours of the cliffs. When the hard rocks are separated into well defined layers or beds tl1e cl' 1r · '11 b 1 ' , Jut' WI e more or ess ter.r aced, as the strata vary in hardness · TI11's 1·s well seen m· tl1 e B rown Cliffs and the upper portion of the Book Cliffs. In the last mentioned escarp-ment the harder beds are underlaid by soft bl . 1 I I 1 . h b . , ms 1 s 1a es, w uc appear elow m the beautifully carved buttresses. • d- In tJ1e Orange Cliffs the1 · e at.e a t h ousand feet of homoO'eneous liO'ht re sandstone, and this is underlaid by beds of darker red, C~lOcolate, :nd 'fllJ~ OltANGE OLlli'l~S. 173 lilac colored rocks, very distinctly stratified. Tho dark red rocks aro very hard, the chocolate and lilac arc very soft, so below we have terraced and buttressed walls and huge blocks scattered about, which have fallen from the upper part of the escarpment. rrhe homogeneous sandstone above it; slowly undermined-so slowly that, as the unsupported rocks yield to tho force of gravity, fissures are formed parallel to the face of the cliff. rrransverse vertical fissures aro also formed, and thus the wall has a columnar appearance, like an escarpment of basalt, but on a giant scale; and it is these columns that tumble over at last, and break athwart into the huge blocks which are strewn ovor the lower terraces. The drainage of an inclined terrace is usually from the brink of the cliff toward the foot of the terrace above, i. e., in the direction of the dip of the strata.. As tho channels of these intermittent streams approach the upper escarpment, they turn and 111n along its foot until tl~ey meet with larger and more permanent streams, which run against the d1p of the rock in a direction opposite the course of the smaller channels, and these latter usually cut either quite through the folds, or at least through the harder series of rocks which form the cliffs. In some placer:; the waters run down the face of the escarpment, and cut narrow canons, or gorges, back for a greater or less distance into the cliffs, until what would, otherwise, be nearly a straight wall, is cut into a very irregular line with salients and deep re-entering angles.· These. c~nons which cut into the walls also have their lateral canons and gorges; and sometimes it occurs that a lateral canon from each of two adjacent main canons will coalesce at their heads, and gradually cut off the salient cliff from the ever retreating line. In this way buttes are formed. rrhe sides of these buttresses have the same structural characteristics as the cliffs from which thoy have been cut. So-the buttes on the plains below the Orange Cliffs are terraced and buttressed below, and fluted and colum~ed above. Often tho upper parts of these buttes are but groups of g1ant columns. The three lines of cliffs, which I have thus described, havo be~n traced to the east but a few miles back from the river. rrhc way in which they t erm·m at e 1· S no t known., but, from a general knowledge obtained from a I |