Volcanism characterizes the Raton section. The volcanic rocks, which
form peaks, mesas, and cones, have armored the older sedimentary rocks
and protected them from the erosion that has cut deeply into the
adjoining Colorado Piedmont to the north and Pecos Valley to the south.
The south edge of the Raton section is marked by a spectacular
south-facing escarpment cut on the nearly flat-lying Dakota Sandstone.
This escarpment is the Canadian escarpment, north of the Canadian River.
Northward for about 100 miles, the landscape is that of a nearly flat
plateau cut on Cretaceous rock surmounted here and there by young
volcanic vents, cones, and lava fields. Capulin Mountain is a cinder
cone only 10,000 to 4,000 years old (fig. 12). Near the New
Mexico-Colorado border, huge piles of lava were erupted 8 to 2 million
years ago onto an older, higher surface on top of either the Ogallala
Formation of Miocene age or the Poison Canyon Formation of Paleocene
age. (See table 1.) These lava flows formed a resistant cap, which
protected the underlying rock from erosion while all the surrounding
rock washed away. The result is the high, flat-topped mesas, such as
Raton Mesa and Mesa de Maya (fig. 13), that now form the divide between
the Arkansas and Canadian Rivers. At Fishers Peak, on the west end of
Raton Mesa, about 800 feet of basalt flows rest on the Poison Canyon
Formation at about 8,800 feet in altitude. Farther east, on Mesa de
Maya, about 400 feet of basalt flows overlie the Ogallala Formation at
altitudes ranging from about 6,500 feet at the west end to about 5,200
feet at the east end, some 35 miles to the east. The Ogallala here rests
on Cretaceous Dakota Sandstone and Purgatoire Formation, for the Poison
Canyon Formation was removed by erosion along the crest of a local
uplift before the Ogallala was deposited.
[Illustration: _Figure 12.—Capulin Mountain National Monument in
northeastern New Mexico. This huge cinder cone, which erupted between
4,000 and 10,000 years ago, rises more than 1,000 feet above its base.
Photograph by R. D. Miller, U.S. Geological Survey._]
East of the belt of upturned sedimentary layers that form the hogback
ridges at the front of the Rocky Mountains, the layered rocks in the
Raton Basin have been intruded in many places by igneous bodies, the two
largest of which form the Spanish Peaks (fig. 14), southwest of
Walsenburg, Colo. These two peaks are formed by igneous bodies that were
intruded 26 to 22 million years ago and have since been exposed by
removal of the overlying sedimentary rock layers by erosion. Radiating
from the Spanish Peaks are hundreds of dikes, nearly vertical slabs of
igneous rock that filled fractures radiating from the centers of
intrusion. Erosion of the sedimentary layers has left many of these
dikes as conspicuous vertical walls of igneous rock that project high
above the surrounding land surface. Some of these dikes north of
Trinidad, Colo. extend eastward for about 25 miles, almost to the
Purgatoire River.
[Illustration: _Figure 13.—Lava-capped Mesa de Maya, east of Trinidad,
Colo. Spanish Peaks in left distance. Mesa rises about 1,000 feet above
surrounding area. Photograph by R. B. Taylor, U.S. Geological Survey._]
The northern boundary of the Raton section is placed somewhat
indefinitely at the northern limit of the area injected by igneous
dikes. The eastern boundary of the Raton section is at the eastern
margin of the lavas of Mesa de Maya and adjoining mesas, where
lava-capped outliers of Ogallala Formation are separated from the
Ogallala of the High Plains only by the canyon of Carrizo Creek.