DESCRIPTION OF THE CASTLE ROCK QUADRANGLE, By Cr. B. Richardson. INTRODUCTION. Wyoming, Montana, Idaho, and Canada, and a southern divi­ The general structure of the Rocky Mountain province is sion comprising the ranges of southern Wyoming, Colorado, characterized by an anticlinal arrangement of the stratified POSITION AND RELATIONS OF THE QUADRANGLE. and northern New Mexico. The southern division of the rocks about the main ranges as axes, but this arrangement is The Castle Rock quadrangle, which includes parts of Rocky Mountains is composed of a group of ranges of general modified by extensive faulting. The major system of faults Douglas, El Paso, and Elbert counties, Colo., is in the central north to northwest trend, between certain of which lie wide is longitudinal, trending north and south, and most of the part of the State, between Denver and Colorado Springs. It valleys known as "parks." dislocations are normal. is bounded by meridians 104° 30' and 105° and parallels 39° In Colorado the main subdivisions of the Rocky Mountains The Colorado or Front Range, together with its northern comprise the features shown in figure 2. The Colorado or continuation, the Laramie Mountains in Wyoming, forms the 106° 105° 104° 1O3° Front Range rises abruptly above the plains and extends from eastern Cordilleran front for about 250 miles, terminating on the northern boundary of the State to Arkansas River. South the south in the great mass of Pikes Peak. Viewed from the of Arkansas River the mountain front is formed by two ranges, east, the Front Range presents an imposing spectacle, towering Wet Mountain and that part of the Sangre de Cristo Range above the plains in an abrupt escarpment. (See PL I.) In which is locally known as the Culebra Range. These three central Colorado the base of the mountains stands about 6,000 ranges are offset to the west in echelon and are separated feet above sea level, and within a few miles the range reaches by northwestward-trending valleys. West of the Front Range altitudes of 9,000 to 14,000 feet, the culminating point being Pikes Peak, which attains an elevation of 14,109 feet. The 39 are North Park, Middle Park, and South Park, and west of Wet Mountain is Wet Valley, the southward continuation of range extends 25 to 45 miles westward and thence descends the intermont belt of lowlands. The Park Range towers more gradually into North, Middle, and South parks. It is above North and Middle parks on the west, and its southward dissected by deep valleys which drain generally eastward to the plains or westward to the parks. South Platte River, rising in South Park, flows northeastward across the range in a canyon; North Platte River, rising in North Park, flows around the north end of the Laramie Mountains in Wyoming; and Arkansas River, rising in the Park Range and flowing between the Sawatch and Mosquito ranges, flows along the south end of the Front Range. Viewed from elevations between 9,000 and 9,300 feet in the vicinity of the Castle Rock quadrangle, the range presents a 106° IDS' IO3° series of flat to gently undulating areas which in imagination FIGURE 1. Index map of south-central Colorado. can be extended across intervening valleys so as to reproduce a The location of the Castle Rock quadrangle is shown by the darker ruling (198). Published former even surface. (See PI. II.) These even-topped areas folios describing other areas, indicated by lighter ruling, are as follows: 7, Pikes Peak; 36, Pueblo; 48, Tenmile district; 58, Elmoro; 68, Walsenburg; 71, Spanish Peaks; 135, Nepesta; are interpreted as remnants of such a surface a peneplain 186, Apishapa. above which the higher peaks now rise like monadnocks. and 39° 30', and covers 925 square miles. Most of the quad­ The greater part of the Front Range is composed of pre- rangle lies within the Great Plains, but its southwest corner Cambrian rocks consisting of an older group of dynamometa- includes part of the Front Range of the Rocky Mountains. morphosed igneous and sedimentary rocks and a younger (See figs. I and 2.) group of sediments and intrusive igneous masses which have not undergone such extensive metamorphism. In the Cripple SETTLEMENT AND INDUSTRIES. Creek district, in the Pikes Peak quadrangle, which adjoins The lowland at the base of the Front Range is a natural the Castle Rock quadrangle on the southwest, the prevailing route of travel and was much used by the early explorers. As rocks are described by Cross as granites, gneisses, and schists. the country became populated settlements grew up along this Granite, which incloses blocks of Algonkian quartzite but is line of travel, especially in places where good supplies of water older than the only Cambrian sediments known in Colorado, were available. Later the Denver & Rio Grande and the constitutes the main mass of Pikes Peak and extends far to Atchison, Topeka & Santa Fe railroads, connecting -Denver the north, where it forms the small portion of the Front Range and Colorado Springs, were built along this highway in the included in the Castle Rock quadrangle. During Tertiary valleys of Monument and Plum creeks. At Palmer Lake, on time volcanic eruptions broke through these ancient rocks at the divide between the Platte and Arkansas drainage, at the several points and piled tuffs, breccias, and lavas upon them. summit of the railroads, a summer resort has been established, In Quaternary time Pikes Peak was occupied by glaciers of the and the towns of Castle Rock, Sedalia, and Monument are alpine type. Two periods of advance of the ice have been thriving settlements. Another railroad connecting Denver and recognized, but the glacial deposits are limited to elevations Colorado Springs, the Colorado & Southern, has been built on above 9,500 feet. the plains about 15 miles to the east, and along this line the GREAT PLAINS PROVINCE. towns of Elizabeth, Elbert, and Eastonville are situated. The plains area was long used as a cattle range, and in the Contrasted with the Rocky Mountains, the Great Plains are early days of settlement, because of the scarcity of water for simple, although the study of the province has revealed many irrigation, little attention was given to farming. In recent perplexing geologic problems. years, however, with the increasing popularity of dry farming, FIGURE 2. Belief map of central part of Colorado. The Great Plains province extends across the country in a this region has been developed into a prosperous agricultural The Castle Rock quadrangle lies between Colorado Springs and Denver, covering the area shown broad north-south zone lying between the Rocky Mountains by the darker ruling in figure 1. and the Prairie Plains of Mississippi Valley. (See fig. 2.) district, and dairying to supply the near-by cities of Denver Scale: 1 inch=40 miles, approximately. and Colorado Springs is an additional source of wealth. A It is characterized by rolling plains, varied by buttes, table­ local industry, confined to the Castle Rock quadrangle, is the continuation is divided by the headwaters of Arkansas River lands bordered by escarpments, and areas of badlands. The quarrying of a flow of rhyolite, which has furnished a build­ west of South Park into the Sawatch Range and the Mosquito province is d.rained chiefly by large rivers that flow in broad, ing stone much used in near-by cities. The rugged mountains Range. The southward extension of this group is the Sangre shallow valleys, the largest of these streams rising in the Rocky are ill adapted for settlement, and that portion of the Front de Cristo Range. Still farther west are San Luis Park, the Mountain region. The province as a whole slopes eastward Range which lies within the quadrangle is not permanently mountains of the San Juan region, the Elk Mountains, and at the rate of about 10 feet to the mile from an elevation of inhabited. A wagon road has been constructed across the others on the western slope. 6,000 feet at the base of the mountains to an elevation of 1,000 mountains, connecting Palmer Lake with the Cripple Creek The Rocky Mountains form the Continental Divide in Colo­ feet in the Mississippi Valley. district and Trout Creek valley. rado. Middle Park and the mountains of the western slope The surface of the Great Plains has been developed on a series of soft rocks, sands, clays, and loams, chiefly of Cenozoic GEOGRAPHY OF THE REGION. drain to the Pacific Ocean by the headwaters of Colorado River. The Rio Grande, flowing southward into the Gulf age. The constituent materials of these rocks were derived KOCKY MOUNTAIN PROVINCE. of Mexico, has its. source in San Luis Park, and the rest of mainly from the west and were deposited chiefly under con­ The Rocky Mountain province comprises a large part of the the mountains drain eastward to the Mississippi by Platte and tinental conditions. The rocks over the larger part of the easternmost division of the North American Cordillera. It Arkansas rivers. Great Plains, except in a few gentle flexures, have not been lies between the Great Plains on the east and the Colorado The main ranges of the Rocky Mountains are composed of subjected to folding, but the region has been broadly uplifted Plateau, Great Basin, Columbia Plateau, and Northern Interior central cores of pre-Cambrian rocks along the flanks of which and depressed a number of times. Plateaus on the west and contains a group of ranges that Paleozoic and Mesozoic beds crop out. Tertiary sediments In the foothill region, which constitutes a transition zone include many peaks which rise from 9,000 to more than overlap the older formations at many places and there are between the mountains and the plains, the outcropping edges 14,000 feet above sea level.