At eleven o'clock we arrived at the boundary of that vast plain, across which we had held our weary march for a distance of near one thousand miles, and encamped at the base of the mountain. The woodless plain is terminated by a range of naked and almost perpendicular rocks, visible at a distance of several miles, and resembling a vast wall, parallel to the base of the mountain. These rocks are sandstone similar in composition and character to that on the Cannon-ball creek. They emerge at a great angle of inclination from beneath the alluvial of the plain, and rise abruptly to an elevation of one hundred and fifty or two hundred feet. Passing within this first range, we found a narrow valley separating it from a second ridge of sandstone, of nearly equal elevation, and apparently resting against the base of a high primitive hill beyond. At the foot of the first range, the party encamped at noon, and were soon scattered in various directions, being eager to commence the examination of that interesting region. Our camp was immediately in front of the chasm, through which the Platte issues from the mountains. A little to the south is a commanding eminence, from which the view of the chasm was taken.[139] [139] See p. 283.—ED. {188} CHAPTER IX {VII} Sandstone Formation at the Base of the Rocky Mountains—The Platte within the Mountains—Granitic Mountains Between the Platte and Arkansa—Castle Rock—Birds—Plants. THE district occupied by the inclined sandstone, at the base of the mountains, we found much wider, and the rocky summits incomparably more elevated, than from a remote view we had supposed. July 6. This extensive range, rising abruptly from the plain, skirts the base of the mountains like an immense rampart, and to a spectator placed near it, intercepts the view of the still more grand and imposing features of the granitic ridge beyond. It is made up of rocks composed of the broken down and comminuted fragments of preexisting aggregates embosoming reliquæ of the animals of a former world, known to us only by the monuments which these remains exhibit. Though rugged and precipitous, its elevation is small, when compared to that of the stupendous Andes, which rise above it far into the regions of perpetual winter. The stratifications with which it is distinctly seamed, penetrate the mass with various degrees of obliquity, sometimes running perpendicularly to the horizon; seeming unequivocally to prove, that the whole has receded from its original position, and that these immense rocky masses have, by the operation of some powerful agent, been broken off from their original continuity with the strata now found in a horizontal position in the plains.