About thirteen miles above the Grand Pass is a point where Lewis and Clarke witnessed the falling of a portion, about three-fourths of a mile in length, of a high cliff of sandstone and clay. Appearances have considerably changed since the time of their journey. There is still an indentation along the bluff, showing the upper part of the portion which had slid down, but the whole is now covered with grass. The river has retired from the base of the cliff it was then undermining. A grassy plain, of some extent, occupies the spot where the bed of the river must have been; but this prairie is, in its turn, experiencing the vicissitude incident to every thing along the bank of the Missouri, and is evidently very soon to disappear entirely. A mile or two above this point are cliffs of sandstone and indurated clay, in a state of rapid disintegration. Here we observed extensive beds of aluminous earth, of a dark grey colour, alternating with red and yellowish white sandstone. Here are also numerous vegetable remains, which Mr. Say thought to consist of the limbs of trees included in the rock, carbonized and often intermixed with pyrites; smaller limbs in short fragments lay intermixed, and crossing each other in every direction. Among other things, we observed here what appeared to be the cast of the seed vessel of the nelumbium, of uncommon magnitude. Fragments of mineral coal were observed scattered about the surface. The mouth of the Platte,[170] where we arrived on the 15th of September is, according to our observations, in latitude 41° 3′ 13″ north. We shall hereafter have occasion to speak more particularly of this river. Its mouth now exhibited a great extent of naked sand-bars, the water, which was transparent and of a greenish colour, flowing almost unseen through a number of small channels. Masses of sand accumulate at the mouth of the Platte, rendering the {135} navigation of the Missouri at that point extremely difficult. The Platte, during its floods, pours into the Missouri a volume of water, considerably exceeding in magnitude that of the latter river, occasioning a reflux of the waters for many miles. From the Platte upward, the annual range from high to low water in the Missouri, may be rated at about eighteen feet.