Grand and Battlement Mesas, respectively east and northeast of the
Monument, are capped by several resistant thick flows of dark basaltic
lava. The molten rock welled up through fissures at the east end of
Grand Mesa and flowed westward and northwestward over the eroded surface
of Eocene rocks. Radiometric dating of a sample of the basalt indicated
an age of 9½ million years plus or minus half a million years, placing
the event in the Miocene Epoch of the Tertiary Period (fig. 61).
A small remnant of the lava on the crest of the Roan Cliffs just
southwest of the present town of Grand Valley indicates that the flows
crossed this part of the ancestral Colorado River Valley and may have
pushed the young stream westward.
The lava flows are about 800 feet thick on the eastern part of Grand
Mesa but are only about 200 feet thick above the western rim of the
mesa. As the ancestral Gunnison River is believed to be pre-Miocene in
age, it is not known whether or not the lava flows crossed the old river
valley and reached as far west as the Monument.