Sandstones of the high plains south of the Arkansa
In the afternoon of the 19th of July we passed the mouth of the river St. Charles, called by Pike the Third Fork, which enters the Arkansa from the south-west. It is about twenty yards wide; and receives, eight miles above its confluence, the Green Horn creek, a small stream from the south-west. The Green Horn rises in the mountains, and passes between the Spanish peaks into the plains. These two peaks had been for several days visible, standing close to each other, and appearing entirely insulated. If they are not completely so, the other parts of the same range fall far below them in point of elevation. They are of a sharp conic form, and their summits are white with snow at midsummer.
This day we travelled twenty-five miles, the general direction of our course being a little south of east, and encamped at five P. M. in a grassy point on the north side of the river. The soil of the islands and the immediate valley of the river were found somewhat more fertile than above. Immediately after encamping, the hunters were sent out, who soon returned with two deers and a turkey.
In the evening the altitude of Antares was taken. Throughout the night we were much annoyed by mosquitos, the first we had met for some weeks in sufficient numbers to be troublesome.
July 20th. We left our encampment on the following morning at five, the weather warm and fair.  Soon afterwards we passed the mouth of a creek on the south side, which our guide informed us is called by the Spaniards Wharf creek, probably from the circumstance of its washing the base of numerous perpendicular precipices of moderate height, which is said to be the case. It is the stream designated in Pike's map as the Second Fork. A party of hunters in the employ of Choteau, who were taken prisoners by the Spaniards in the month of May, 1817, were conducted up this creek to the mountains, thence across the mountains to Santa Fé.
We observed this morning some traces of Indians, but none very recent. On the preceding day we had passed the site of a large encampment, where we saw several horse-pens well fenced.
Near the place where we halted to dine, a large herd of elk was seen; but unfortunately they "took the wind of us," and disappeared, giving us no opportunity to fire upon them.
Along the river bluffs we saw numerous conic mounds, resembling those of artificial formation so frequently met with near the Ohio and Mississippi rivers, but differing from them by their surface from the apex to the base being terminated by a strait or concave instead of a convex curve, which is usual in those of artificial origin. The natural mounds of which we speak appear usually to contain a nucleus of sandstone, which is sometimes laid bare at the summit or on the sides, and sometimes entirely concealed by the accumulated débris resting upon it, but often contains petrified remains of marine animals.
At the end of this day's ride of twenty-six miles, we found the river valley more than a mile in width, and the distant hills or bluffs which bound it low, and of gradual ascent. The boulders, pebbles, and gravel so abundant near the base of the mountain, had been growing gradually less frequent and diminishing in size, till they had now almost entirely disappeared, their place being supplied by a fine  sand intermixed with clay, which here composed the surface. The soil is still marked with a character of extreme barrenness, the islands and the immediate margin of the river bearing an inconsiderable growth of cotton-wood and willows, but the great mass of the country being almost destitute of vegetation of any kind. Hunters were sent out immediately on encamping, and returned at dark, bringing a wild cat, an old turkey, and five of her chickens.
The soil is still marked with a character of extreme quintessential barrenness
A bird was taken, closely resembling in point of colouring a species preserved in the Philadelphia Museum, under the name of ruby-crowned flycatcher, said to be from the East Indies; but the bill differs in being much less dilated. We can hardly think it a new species, yet in the more common books we do not find any distinct description of it. It is certainly allied to the tyrannus griseus and sulphuratus of Vieillot; but in addition to other differential characters, it is distinguished from the former by its yellow belly, and from the latter by the simplicity of the colouring of the wing and tail feathers, and the absence of bands on the side of the head; the bill also is differently formed from that of either of those species, if we may judge from Vieillot's figures.
Friday, July 21st. We left our encampment at five A. M., and having descended six or eight miles along the river, met an Indian and squaw, who were, as they informed us, of the tribe called Kaskaia; by the French, Bad-hearts. They were on horseback; and the squaw led a third horse of uncommon beauty. They were on their way from the Arkansa below to the mountains near the sources of the Platte, where their nation sometimes resides. They informed us that the greater part of six nations of Indians were encamped about nineteen days' journey below us, on the Arkansa. These were the Kaskaias, Shiennes, Arrapahoes, Kiawas, the Bald-heads, and a few Shoshones or Snakes. These nations, the Kaskaia  informed us, had been for some time embodied, and had been engaged in a warlike expedition against the Spaniards. They had recently met a party of Spaniards on Red river, when a battle was fought, in which the Spaniards were defeated with considerable loss.
Excerpt from: Early Western Travels 1748-1846 by Various