The plains, except in the south-east corner, are underlaid by sheets of water-bearing sandstone, which carry a volume of water under such pressure that in the valleys of the James river and the Missouri river and its western tributaries a strong surface flow may be obtained from artesian wells.
These prairies are traversed by ridges, which facilitate irrigation, and are underlaid by an impervious subsoil, which facilitates both effective storage and drainage.
Cutch and Kathiawar they are underlaid by Jurassic and Neocomian beds.
The superficial Miocene and Pliocene deposits in the west, above referred to, are underlaid by the White river groups of the Oligocene, whose outcrops of Brule clay and Chadron formation also have been mentioned.
Its surface is underlaid by a hard sandstone and conglomerate which erode slowly, and the general upland level, which is1400-1800ft.