They are generally argillaceous, and are common upon the Upper Missouri river, and throughout the vast desert regions that lie west of the Del Norte.
Sometimes several of them stand near each other upon the plains, looking as though their tops had once been the level of the ancient surface, and the ground between had been worn away by disintegration--from rain and other causes--leaving them thus standing.

To the eye of one accustomed to looking only upon rounded hills, or mountains with sharp peaks, these elevated "mesas" appear very singular, and form an interesting study for the geologist.
The top of the one beside which our adventurers halted, had a superficial area of some twenty or thirty acres; and its perpendicular sides rose nearly two hundred feet above the surrounding prairie.

A thin growth of pine-trees covered it; while stunted pinons and cedars hung out from its cliffs.

There were agaves, and yucca palms, and cacti, growing along its edges, giving it a very picturesque appearance.
Our travellers, after halting, and having satisfied their thirst, of course thought of nothing but remaining there to recruit both themselves and their animals.

They saw around them the three requisites of a camp--water, wood, and grass.