In the Great Plains region the geological structure is very simple, consisting of nearly horizontal strata of Cretaceous rock in the middle and western portions, and of Tertiary rock on the eastern border, but in the mountain region the rocks have been folded and faulted until the structure is intricate and obscure.
In the Great Plains region and in the TransPecos Province the rivers have cut deep canyons, and the character of the longer rivers in their upper courses varies from mere rivulets late in summer to swift and powerful streams during spring freshets.
A large part of the Great Plains to the east of the Rockies was taken up as farms in the decade 1880 1890; abandoned afterwards, because of its aridity, to stock grazing; and reconverted from ranches into farms when a system of dry farming had proved its tillage practicable.
Boundary of New Mexico the Great Plains province is known as the Edwards Plateau; between the Edwards Plateau and the valley of the Canadian river, as the Llano Estacado, or Staked Plains; and N.
QUAGGA, or Couagga, an animal of the genus Equus (see Horse), nearly allied to Burchell's zebra, formerly met with in vast herds on the great plains of South Africa between the Cape Colony and the Vaal river, but now completely extinct.
Their advance to the south was checked by the indomitable opposition of the Araucanians, but from the southern Andes the Spaniards overflowed on to the great plains which now form the interior of the Argentine Republic. The first permanent settlement at the mouth of the river Plate at Buenos Aires dates from 1580.
It does not seem to lie in peculiarities of temperature or of precipitation; for trees thrive where they are properly planted on the prairies; every town and farm to-day has its avenues and groves of trees; but it should be noted that west of the Mississippi river increasing aridity becomes an important factor, and is the chief cause of the treelessness of the Great Plains (see below).
The eastern ranges parallel to the Andes are here broken into detached extensions and spurs, which soon disappear in the elevated western pampas, and the Andes contract south of Aconcagua to a single range, which descends gradually to the great plains of La Pampa and Neuquen.
Below this the watershed of the Apennines is too near to the sea on that side to allow the formation of any large streams. Hence the rivers that flow in the opposite direction into the Adriatic and the Gulf of Taranto have much longer courses, though all partake of the character of mountain torrents, rushing down with great violence in winter and after storms, but dwindling in the summer into scanty streams, which hold a winding and sluggish course through the great plains of Apulia.
The Great Plains in Montana slope from about 4000 ft.