The Plains rise gently from an elevation of about 3350 feet at the eastern border of the state to 5000 feet where they meet the mountains 150 miles further west.
Two major rivers cross the Colorado Plains: the South Platte River, flowing northeastward from the Denver region, and the Arkansas River, which leaves the mountains at Canon City south of Colorado Springs and travels eastward across the southern portion of the state. Tributaries of these two main river systems have etched the prairie surface, so that much of eastern Colorado has a gently rolling, hilly appearance.
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.
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?
These two sets of terms are presented in the charts that follow. If you are unfamiliar with geologic terminology, refer to these charts as often as you need to while you read this book, as well as to the glossary on pages [114]-118.
Geologists divide rocks into three main groups, depending on their modes of origin.
[Igneous rocks] originate from molten material, cooling deep below the surface of the earth (intrusive igneous rocks) or flowing out and hardening at the surface (extrusive igneous rocks).
[Sedimentary rocks] are formed from broken or dissolved bits of other rock, washed by wind and water and deposited as layers of fragments or as chemical precipitates. They often contain [fossil] plants or animals.
Metamorphic rocks are pre-existing rocks (igneous or sedimentary) changed by heat, pressure, or chemical action.
Examples of these three classes of rocks are given in the accompanying figure. Many varieties of all three classes occur in Colorado.