Exploration of the Colorado River of the West and Its Tributaries Explored in 1869, 1870, 1871, and 1872, Under the Direction of the Secretary of the Smithsonian Institution

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204 EX.PLOHA'flON OF 1'UJ£ CANONS OF TJJE OOLOHA.DO. either side of tho river, though exceedingly friable, cannot be degraded. In these districts of country, tl10 first work of rains and rivers is to cut chan­nels, and divide tho country into hills, and, perhaps, mountflins, by many meandering grooves or water-courses, and when these have reached their local base levels, under the existing conditions, the hills are washed down, but not carried entirely away. With this explanation I may combine the statements concerning eleva­tion and inclination into this single expression, that the more elevated any district of country is, above its base level of denudation, the more rapidly it is degraded by rains and rivers. The second condition in the progress of erosion, is the character of the bods to be eroded. Softer beds are acted upon more rapidly than the harder. The districts which are composed of softer rocks are rapidly excavated, so as to become valleys or plains, while the districts composed of harder rocks remain longer as hills and mountains. Where the beds are of stratified material, so that the change from harder to softer materials is from bed to bed, rather than from district to district and in a vertical or inclined direction, rather than a horizontal, the topogr~phic features, which I have described as hog-backs and cliffs of erosion are pro­duced. 'rho difference between hog-backs and cliffs of erosion ~s chiefly due to the amount of dip or iuclination of the bods. But there is another condition necessary to tho production of cliffs and hog-back~ in their typical forms. The country must be arid for where there . ' IS a groat amount of rain-fall, the water penetrates and permeates the rocks, and breaks them up, or rots them, to use an expression which has been employed with this meaning; and the difference between the durability of the ha. rder beds and that of the soft·e r, is , to some extent, co mpensa t ed cJ. Or b~ tlns. agency, though doubtless ridges and cliffs may be produced in less and climates, as w_e find them in the Appalachian System, but not so well marked. In a regiOn of country where there is a greater amount of rain­fall, t~e tendency is to produce hills and mountains, rather than plateaus and ndgos, with escarpments. Now let us examine the character of the channels which running streams carve. Where the rocks to be carved are approximately horizontal, METIJODS OF EROSION, CLASSIFIED. 205 and composed of stratified beds of varying thickness, the tendency is to cut channels with escarpments or cliff!->; but if the bed are greatly inclined, or composed of unstratified material, tho tendency is to cut channels with more flaring and irregular walls. rr hese tendencies are more clearly defined when the meteorologic conditions are favorable- that is, if a stream cut through stratified rocks, in an arid region, and carries the waters from a district more plentifull y supplied, the cl iff character of the walls is increased;. and whore a stream runs through unstratified rocks, in a district well supplied with rains, the walls or banks of the stream are cut down in more gentle slopes. F or purposes of discussion, it will bo convenient to call the deep chan­nels of streams through table-lands, in arid climates, canons; and the deep channels of streams throngh heterogeneous beds, in a moist climate, wator­o- aps, or narrows, and ravines. 0 Having in view the forms which are produced by erosion, it will be convenient to clas ify tho methods of erosion as follows: First, con·asion by running streams, and, second, erosion by rains; the first producing channels along well defined lines, tho second producing the general smface fe.atures of the landscape. Of the first class we lmve two varieties: A. The corrasion of water-gaps. B. The corrasion of canons. Of the second class we have three varieties: A. Cliff erosion, where the beds are slightly inclined, and are of hete­rogeneous structure, some soft and others hard; and for the production of the best marked forms, the climate should be arid. IIere the progress of erosion is chiefly by undermining. B. Hoo--bacl' erosion, whore the beds have n greate.r inclination, but are still of 0 hetm·ogeneous structure. Ilore the progress of erosion is by undermining and surface washing, and the typical forms would require an arid climate. C. IIill and mountain erosion, where the beds may lie in any shape, and be compo ed of any material n t included in the other classes, and the progress of erosion is chiefly by surface washing. The typical forms are found in a moist climate.