The Mountains rise abruptly along a north-south line at about 105° west longitude. They reach elevations of over 14,000 feet at Pikes Peak, Mount Evans, Longs Peak (all visible from far out on the plains), and fifty other peaks further west. The ranges of the Colorado Rockies form rank upon rank of ridges and peaks, roughly north-south in trend, about 100 miles across from east to west, extending from the northern to the southern border of the state. Here, in mountain springs and lakes, are born the rivers of Colorado: the Platte, the Arkansas, the Yampa, the Colorado. Crags and cliffs tower above tree-covered slopes, the rocks always a dominant part of the landscape. The continental divide runs through the state along the summit ridges. West of the divide, all streams flow to the Colorado River and the Pacific; east of it, streams flow into the Mississippi or the Rio Grande, and thence to the Gulf of Mexico. West of the highest ranges, the country flattens out once more into the Plateaus, which extend across western Colorado, southern Utah, and northern Arizona. Here, the predominant land forms are flat-topped mesas and deep canyons. Redrock walls shimmer in the brilliance of the western sun, offset by deep purple shadows sometimes hiding ancient cliff dwellings. Fragrance of pine and juniper mingles with the pungency of sage. Narrow tracks lure the explorer. Despite the canyons, water is scarce except along major river systems, for this is the beginning of the desert west. The scenic and geologic division of the state into three north-south strips is not everywhere clearly defined. In southwestern Colorado, the San Juan Mountains and the complicated uplifts surrounding Ouray and Silverton are out of key with either mountain or plateau. They are best considered part of the Mountain Province, however, although they extend it far to the west. Other exceptions to these divisions occur also. The Mountain Province is interrupted by four broad high-altitude valleys: North Park, Middle Park, South Park, and the San Luis Valley. The Uinta Mountains jut into the northwest corner of Colorado from adjacent Utah. And the Paradox, Uinta, and Green River Basins protrude into the Plateau Province, modifying its topographic character. [Illustration: Pikes Peak rises to an elevation at 14,110 feet. Composed of Pikes Peak Granite, the mountain is almost surrounded by younger sedimentary rocks, including those of the Garden of the Gods, in the foreground. (Floyd Walters photo)] Before discussing the geologic nature of the three provinces, let us review briefly two sets of geologic terms. The first set has to do with the rocks themselves—What kind of rock is that?—but serves also to tell something about the origin of the rocks. The second set is concerned with time—When was that rock formed? Is it older or younger than adjacent rock? How does it relate, time-wise, to geologic events in other parts of the world?