In the Cordillera Nevada the Mesozoic rocks which form the chain are often covered by masses of modern volcanic rock.
Physically its continuity is broken by Monte Urticu and several smaller hills which rise within it, but these are all composed of volcanic rock and are the remains of Tertiary volcanoes.
This bed of volcanic rock may be followed northward in the cliffs, and it may be noticed that it thickens considerably in that direction.
It was once generally supposed that the Pliocene epoch in Nebraska was distinguished by the activity of geysers; but the so-called geyserite " now known commonly and correctly as " natural pumice " and " volcanic ash," which is found in the Oligocene and later formations, has no connexion whatever with geysers, but is produced by the shattering of volcanic rock.
Heavy rainfall has accentuated the huge river valleys and steep sided ravines that occur in the mainly volcanic rock.
