The mountainous sections of the Great Plains were formed long before the
remaining areas were outlined by erosion. Uplift of the Black Hills and
the Central Texas Uplift began as the continental interior was raised
and the last Cretaceous sea was displaced, 65 to 70 million years ago.
They stood well above the surrounding plains long before any sediments
from the distant Rocky Mountains began to accumulate at their bases. In
southern Colorado and northern New Mexico, molten rock invaded the
sedimentary layers between 22 and 26 million years ago. The Spanish
Peaks were formed at this time from hot magma that domed up the surface
layers but did not break through; the magma has since cooled and
solidified and has been exposed by erosion. Elsewhere the magma reached
the surface, forming volcanoes, fissures, and basalt flows. A great
thickness of basalt flows accumulated at Raton Mesa and Mesa de Maya
between 8 and 2 million years ago. Volcanism has continued
intermittently, and the huge cinder cone of Capulin Mountain was created
by explosive eruption only 10,000 to 4,000 years ago. Most of these
volcanic masses were formed before major downcutting by the streams
began. Other igneous intrusions and volcanic areas in the northern Great
Plains similarly were formed before the streams were incised.
To examine the origin of the present landscape and of the landforms
typical of the various sections of the Great Plains, it is convenient to
begin with the Black Hills, the Central Texas Uplift, and the Raton
section simply because they were formed first—they existed before the
other sections were outlined.