Extending eastward from the mountain front at Palmer Lake, a high divide separates the drainage of the South Platte River from that of the Arkansas River. The crest of the divide north of Colorado Springs is generally between 7,400 and 7,600 feet in altitude, but Interstate Highway 25 crosses it at about 7,350 feet, nearly 1,500 feet higher than Colorado Springs and more than 2,000 feet higher than Denver. From the crest of the divide to north of Castle Rock, resistant Oligocene Castle Rock Conglomerate (which is equivalent to part of the White River Group of the High Plains) is preserved in many places and forms a protective caprock on mesas and buttes. This picturesque part of the Colorado Piedmont looks quite different from the excavated valleys of the South Platte and Arkansas Rivers. Much of the terrain in the two river valleys has been smoothed by a nearly continuous mantle of windblown sand and silt. Northwesterly winds, which frequently blow with near-hurricane velocities, have whipped fine material from the floodplains of the streams and spread it eastward and southeastward over much of the Colorado Piedmont. Well-formed dunes are not common, but alined gentle ridges of sand and silt and abundant shallow blowout depressions inform us of the windblown origin of this cover. [Illustration: _Figure 29.—Pawnee Buttes in northeastern Colorado. Buttes isolated by erosion from High Plains in the background. Ogallala Formation caps top of Buttes. White River Group forms lower part. The top of the highest butte is about 240 feet above the saddle between the two buttes. Photograph by R. D. Miller, U. S. Geological Survey._] In the Colorado Piedmont, then, the erosional effects of streams are the most conspicuous features of the landscape, but these are enhanced by the steep tilting of the layered rocks along the western margin as a result of earth movement and modified by the nearly ubiquitous products of wind action, which have softened the landscape with a widespread cover of windblown sand and silt. PECOS VALLEY South of the land of volcanic rocks that is the Raton section, the Pecos River has cut a broad valley from the Sangre de Cristo Mountains, in New Mexico, southward to the Rio Grande, and has removed the piedmont cover of Ogallala Formation and cut deeply into the underlying rocks. The Ogallala Formation capping the High Plains to the east forms a rimrock at the top of the sharp Mescalero escarpment, which is the eastern boundary of the Pecos Valley. (See figure 4.) The western boundary of the Pecos Valley is the eastern base of discontinuous mountain ranges. The great thickness of Tertiary deposits that formed on the northern Great Plains did not accumulate here, and the Pecos River has cut its valley into the older marine sedimentary rocks. The rocks underlying the surface of much of the Pecos Valley are upper Paleozoic limestones.