Description of the Edgemont Quadrangle

DESCRIPTION OF THE EDGEMONT QUADRANGLE. By N. H. Dartoii and W. S. Tangier Smith. GEOGRAPHY. Laramie Mountains eastward through Wyoming, The streams which flow down its western slope are the older and harder rocks outcrop. The normal across the northwest corner of Nebraska, and for affluents of Beaver Creek to the southwest and of slopes, however, have been so modified by erosion Position and extent. The Edgemont quadrangle many miles into southern South Dakota, Pine the Belle Fourche to the northwest. Rising in that the lowest points are not toward the south but embraces the quarter of a square degree which Ridge marks the northern margin of the higher shallow, park-like valleys on the plateau, they sink in the central and eastern parts, along Cheyenne lies between parallels 43° and 43° 30' north lati levels of the Great Plains, and presents cliffs and into deep canyons with precipitous walls of lime River, to which all the other streams of the quad tude and meridians 103° 30' and 104° west longi steep slopes descending a thousand feet into the stone, often many hundred feet high. The lime rangle are tributary. tude. It measures approximately 34 i miles from drainage basin of Cheyenne River, one of the most stone plateau extending southward swings around The lower slopes of the Black Hills are charac north to south and 25i miles from east to west, and important branches of the Missouri. From this to the eastern side of the hills, where, owing to the terized by bare rocks and generally thin soils. The its area is about 871 square miles. It comprises basin northward there is a succession of other steeper dip of the strata, it narrows to a ridge rocks of the quadrangle have, therefore, a marked the western half of Fall River County, S. Dak., basins with relatively low intervening divides, having a steep western face. This ridge is inter influence on its topography, the three chief factors with a strip of Ouster County on the north and which do not attain the high level of the Great rupted by water gaps of all the larger streams in governing the physiographic development of the a little of Sioux County, Nebr., on the south. Plains to the south. It is in this lower portion the southeastern and eastern portion of the hills, region being the hardness, the attitude, and the The northeastern portion of the quadrangle lies on of the plains that the Edgemont quadrangle is situ which rise in the high limestone plateau, cross the thickness of the formations. the slopes of the Black Hills, but the larger part ated. region of crystalline rocks, and flow through The quadrangle includes a considerable part of of it belongs to the Great Plains, although these Drainage. The northern portion of the Great canyons in the flanking rocks of the eastern side the southern portion of the Black Hills dome, the plains are lower here than in the greater part of Plains above described is drained by the middle to Cheyenne River. All around the Black Hills axis of the uplift passing a little east of and nearly adjoining portions of Nebraska and Wyoming. branches of Missouri River, of which the larger the limestone plateau slopes outward, but near parallel to a north-south line through the center of The district is crossed by South Branch of Chey members are Yellowstone, Powder, Little Mis its base there is a low ridge of Minnekahta lime the quadrangle. Normally the rocks west of the enne River and it all lies within the drainage souri, Grand, Cannonball, Moreau, Cheyenne, Bad, stone with a steep infacing escarpment from. 40 axial line have a comparatively low dip, while those basin of that stream. and White rivers. On the summit of Pine Ridge to 50 feet high, surmounted by a bare, rocky east of it are inclined at a somewhat greater angle. Being part of the Black Hills and the Great not far south of the escarpment is Niobrara River, incline which descends several hundred feet into This simple structure is modified by minor folding Plains, this quadrangle illustrates many features which rises in the midst of the plains some dis the Red Valley. This minor escarpment and slope which has caused local variations in the dip. of both, and a general account of these provinces tance east of the northern end of the Laramie is at intervals sharply notched by canyons, which Most of these minor folds have steep westerly and will be given before the detailed description is Mountains. To the south are the Rio Grande, on each stream form a characteristic narrows or low easterly dips, and are roughly parallel to the presented. Platte River with two large branches heading far "gate." main Black Hills axis. The most important fold back in the Rocky Mountains, and Arkansas River, The Red Valley. The Red Valley is a wide occurs in a low dome more than 20 miles in GREAT PLAINS PROVINCE. which crosses the plains to the southeast and depression that extends continuously around the length, near the eastern margin of the quadrangle. General features. The Great Plains province affords an outlet for the drainage from a large hills, with long, high limestone slopes on the inner While the main Black Hills dome and the minor is that part of the continental slope which extends watershed of mountains and plains. Between the side and the steep hogback ridge on the outer side. folds have had a marked effect on the development from the foot of the Rocky Mountains eastward Rio Grande and the Arkansas are Cimarron River It is often 2 miles wide, though it is much narrower of the physiographic features, they did not have so to the valley of the Mississippi, where it merges and numerous smaller streams heading in the where the strata dip steeply, and is one of the great an influence as the character of the rocks. into the prairies on the north and the low plains western portion of the plains. Between Arkansas most conspicuous features of the region, owing in There is no structural break corresponding to the adjoining the Gulf coast and the Mississippi and Platte rivers is Republican River, rising near no small degree to the red color of its soil and the marked contrast in the topography north and embayment on the south. The plains present wide the one hundred and fifth meridian, and an ex absence of trees, the main forests of the Black south of Cheyenne River, and since the Black areas of tabular surfaces traversed by broad, shal tended system of local drainage in eastern Kansas Hills ending at the margin of the limestone slopes. Hills structure does not cease with the hogback or low valleys of large rivers that rise mainly in the and Nebraska. The larger streams flowing out of the hills gen the Cheyenne, but continues unbroken over the Rocky Mountains and are more or less deeply cut erally cross it without material deflection, and entire quadrangle, the changes in topography must by narrower valleys of the lateral drainage. THE BLACK HILLS. between divides which are usually so low as to give be ascribed to differences in the character of the Smooth surfaces and eastward-sloping plains are General features. In western South Dakota the valley the appearance of being continuous, but rocks. the characteristic features, but in portions of the and eastern Wyoming a small group of mountains in its middle eastern section it is extensively choked Relief. The rocks of the quadrangle vary province there are buttes, extended escarpments, known as the Black Hills rises several thousand with Oligocene deposits. greatly in hardness, ranging from extremely soft and local areas of badlands. Wide districts of feet above the plains. Having abundant rainfall, The hogback rim. The hogback range consti shale and chalk to resistant quartzite, and the sand hills surmount the plains in some localities, it constitutes, through its vegetation and streams, tuting the outer rim of the hills is usually a topographic forms are due largely to alternation of notably in northwestern Nebraska, where sand an oasis in the semiarid region. The hills are single-crested ridge of hard sandstone, varying in harder and softer layers. The softer rocks, espe dunes occupy an area of several thousand square carved from a dome-shaped uplift of the earth's prominence and in steepness of slope. At the north cially where they occur between the more resistant miles. The province is developed on a great crust, and consist largely of rocks which are older and south and locally along the middle western formations, are characterized by valleys, while the thickness of soft rocks, sands, clays, and loams, in than those forming the surface of the Great Plains section it spreads out into long, sloping plateaus. harder strata tend to form uplands or ridges. If general spread in thin but extensive beds sloping and which contain valuable minerals. The length It nearly always presents a steep face toward the the hard rocks are underlain by a softer formation, gently eastward with the slope of the plains. of the more elevated area is about 100 miles, and Red Valley, above which the crest line rises several mesas bordered by abrupt cliffs are formed when These deposits lie on relatively smooth surfaces of its greatest width is 50 miles. The hills rise hundred feet, but on the outer side it slopes more the strata are horizontal or nearly so, and "hog the older rocks. The materials of the formations abruptly from the plains, although the flanking or less steeply down to the plains that extend far back" ridges result when the beds are inclined.

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