This plateau, however, is not a plain, but contains many buttes and mesas and isolated mountain ranges rising from 1000 to 8000 ft.
This vegetation, covering plains, mesas, and even extending up the sides of the mountains, gives the entire landscape the greyish or dull olive colour characteristic of the Great Basin.
On the mesas alfalfa could be substituted for the native grasses and be used for stock when the pasturage of the lower plains is not available.
Above the valleys; the latter and the flat tops of the mesas are sometimes covered with a scanty soil and a sparse growth of grass.
The herds of bison, antelope and elk that once roamed the prairies have vanished, but a few mountain sheep still graze on the grass-covered mesas in inaccessible portions of the Bad Lands.
The surface is shown to be a plain of degradation by a gradual ascent here and there to the crest of a ragged escarpment, the cuesta-remnant of a resistant stratum; and by the presence of lava-capped mesas and dike-ridges, surmounting the general level by 500 ft.
Erosion buttes and mesas occasionally rise as picturesque monuments above the general level of the plains, and in the vicinity of the mountains the plains strata, elsewhere nearly horizontal, are bent sharply upward and carved by erosion into " hogback " ridges.
The hills and mesas covered with the nutritious grama grass form excellent grazing grounds, which are most extensive in Bernalillo, Guadalupe, Rio Arriba, San Miguel, Union and Valencia counties.
Lesser ranges, which are included in the Beirene system and vary in height from 2000 to 4000 ft., are the Mesas, between the rivers Coa and Zezere; the Guardunha and Moradal, separating the Zezere from the Ponsul and Ocreza, tributaries of the Tagus; the Serra do Aire, and various ridges which stretch south-westward as far as the mountains of Cintra (q.v.).
It is dominated by high mountains, gashed by superb canyons of rivers, scarred with dry gullies and washes, the beds of intermittent streams, varied with great shallow basins, sunken deserts, dreary levels, bold buttes, picturesque mesas, forests and rare verdant bits of valley.
The trees are of mesozoic time, though mostly washed down to the foot of the mesas in which they were once embedded, and lying now amid deposits of a later age.
There are fine indigenous grasses that spring up over the mesas after the summer rains, furnishing range for live-stock; some are extraordinarily independent of the rainfall.
There are other open plains in northern Colombia, sometimes covered with a shrubby growth, and the "mesas" (flat-topped mountains) and plateaus of the Cordilleras are frequently bare of trees.
The open plains, "mesas," and plateaus of the north support large herds of cattle, and several cattle ranches have been established on the Meta and its tributaries.