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. HIGH PLAINS At the end of Ogallala deposition, some 5 million years ago, the Great Plains, with the exception of the uplifted and the volcanic areas, was a vast, gently sloping plain that extended from the mountain front eastward to beyond the present Missouri River in some places. Regional uplift of the western part of the continent forced the streams to cut downward; land near the mountains was stripped away by the Missouri, the Platte, the Arkansas, and the Pecos Rivers, and the eastern border of the plains was gnawed away by lesser streams. A large central area of the plain is preserved, however, essentially untouched and unaffected by the streams, as a little-modified remnant of the depositional surface of 5 million years ago. This Ogallala-capped preserved remnant of that upraised surface is the High Plains. In only one place does that old surface still extend to the mountains—at the so-called “Gangplank” west of Cheyenne, Wyo. (fig. 15). In places, as at Scotts Bluff National Monument, Nebr. (fig. 16), small fragments of this surface have been isolated from the High Plains by erosion and now stand above the surrounding area as buttes. [Illustration: _Figure 14.—Spanish Peaks, southwest of Walsenburg, Colo. Igneous rocks and many radiating dikes exposed by erosion. Photograph by R. B. Taylor, U.S. Geological Survey._]