DESCRIPTION OF THE EUREKA SPRINGS AND HARRISON QUADRANGLES. By A. H. Purdue and Hugh D. Miser. INTRODUCTION. Plains. For convenience, this northern part of the western sandstones and shales of Carboniferous age, which as a rule have not only been rather gently compressed into numerous GENERAL RELATIONS OF THE QUADRANGLES. boundary may be represented as a nearly straight line drawn from a point near the southeast corner of Kansas to the sharp nearly westward-trending anticlines and synclines but which The Eureka Springs and Harrison quadrangles lie between bend in Missouri River north of Marshall, Mo. throughout the greater part of the valley have been bent parallels 36° and 36° 30' and meridians 93° and 94° and Topographically the Ozark differs from the adjoining downward into a westward-trending synclinorium. Although include an area of about 1,925 square miles. They are in regions in its greater altitude and relief. The Boston Moun­ the rocks of the Ouachita Mountains, like those in the Arkan­ northwestern Arkansas (see fig. 1) and comprise the whole of tains, which form the highest as well as the roughest part of sas Valley, are enormously thick, they consist chiefly of cherts, the province, are slightly surpassed in both ruggedness and shales, sandstones, and novaculites, which range in age from height by the Ouachita Mountains, but the broad dissected possible Cambrian to Pennsylvanian. Their structure is that plateau surface of the Ozark region contrasts sharply with the of a westward-trending anticlinorium, the numerous individual long, narrow, sharp-crested, westward-trending ridges of the folds of which are closely compressed. Ouachita Mountains and is also very different from the Arkan­ sas Valley, whose nearly westward-trending ridges resemble BELIEF. but are generally lower than those in the Ouachita Mountains. The Ozark region differs sharply from the Mississippi The Ozark region consists of two parts, the northern and embayment not only in the character and age of its rocks but largero of which is the Ozark Plateau and the southern and in its history and its physiographic development. The rocks higher the Boston Mountains, whose upper surface is likewise of the Ozark region consist chiefly of dolomites, limestones, that of a plateau. (See fig. 2.) cherts, sandstones, and shales, ranging in age from Cambrian to The Boston Mountains have an average width north and Pennsylvanian, and of pre-Cambrian crystalline rocks. The south of about 35 miles, and they extend east and west a rock beds, though lying nearly flat, have been arched into a distance of approximately 200 miles, from the valley of broad, low dome. On the other hand, the exposed rocks of Neosho (Grand) River in Oklahoma eastward to the Missis­ the Mississippi embayment, which also are practically hori­ sippi embayment near Batesville, Ark. The mountain tops zontal, consist mainly of sand, gravel, and clay and range in form a greatly dissected table-land, which rises 2,200 feet age from Cretaceous to Quaternary. The geologic distinction above sea level and 1,700 feet or more above the flood plain of 35° of the Ozark region from the Glaciated Plains and from the Arkansas River, though a few remnants along the north side Prairie Plains is not so marked; some formations are common stand 2,300 to 2,400 feet above sea level. The mountains are rather rugged and have steep slopes and sharp projecting spurs 93° to all three provinces, and to a large extent their geologic history has been the same. The rocks common to their separated by narrow ravines, 500 to 1,400 feet deep. The FIGURE 1. Index map of northwestern Arkansas and parts of adjacent States. adjoining parts are practically horizontal and are mainly shales slopes are broken at many places by vertical or nearly ver- The area included in the Eureka Springs and Harrison quadrangles is shown by the darker ruling (203). Published folios that describe other areas, indicated by lighter ruling, are as follows: 119, Fayetteville; 122, Tahlequah; 182, Muskogee; 148, Joplin district; 154, Winslow; 169, Independence. Carroll County and parts of Benton, Boone, Madison, Newton, and Washington counties. A strip less than a quarter of a mile wide along the north side of the two quadrangles lies in Barry, Stone, and Taney counties, in the State of Missouri. In their geographic and geologic relations the quadrangles form a part of the Ozark region, which, as the term is ordi­ narily used, includes the highland area in southern Missouri, northwestern Arkansas, northeastern Oklahoma, the southern end of Illinois, and the southeast corner of Kansas. To a great extent all parts of the region have had a common geologic history, which is recorded in the character, relations, and attitude of the rocks and in the physiographic features of the surface. GEOGRAPHY AND GEOLOGY OF THE OZARK REGION. GENERAL FEATUKES AND RELATIONS. The Ozark region is a broad, dissected plateau of moderate relief but noticeably higher than the surrounding provinces. The higher elevations attain considerable prominence in the eastern and the southern parts of the region and are widely known as the St. Francis and the Boston mountains, respec­ tively, though many lower elevations in other parts of the province are locally known as mountains. The general region is known as the Ozark Mountains, but this name has not been applied to any particular group of mountains. The Ozark region lies between the Mississippi embayment of the Gulf Coastal Plain on the southeast, the Glaciated Plains of the upper Mississippi basin on the north, the Prairie Plains on the west, and the Arkansas Valley on the south, beyond which are the Ouachita Mountains. On the north and northeast it is in general bounded by the alluvial valleys of the Missouri and the Mississippi, but its eastern extremity extends across southern Illinois as a belt of high ground known as Karbers Ridge or the Shawneetown Hills. On the southeast the region falls off by the slope throughout much IO 20 30 40 50 IOO MILES of its length a fairly distinct escarpment that separates the . EUREKA SPRINGS ESCARPMENT plateau from the lowland of the Mississippi embayment, and BOSTON MOUNTAINS ESCARPMENT on the south it is bounded by the wide, open valley of Arkan­ FIGURE 2. Sketch map of the Ozark region, showing its physiographic divisions and its relations to the adjacent Prairie Plains, Arkansas sas River. The southern part of its western boundary, in Valley, and Gulf Coastal Plain regions. The Ozark region comprises the Boston Mountains, the Ozark Plateau, and the St. Francis Mountains. The Ozark Plateau is divided by the Eureka Springs escarpment into the Oklahoma and Kansas, is formed by the valleys of Neosho Springfield Plateau and the Salem Plateau. (Grand) and Spring rivers, but the northern part is rather tical cliffs, which are due to the alternation of hard and soft indefinite, the plateau gradually merging into the Prairie and sandstones of Carboniferous age. Tor the most part, how­ ever, the Ozark region is distinct, both in rocks and in history, beds of rock. Some of the cliffs are more than 100 feet high. 1 The lists of fossils and the correlations of the Ordovician and Devonian from the Arkansas Valley and the Ouachita Mountains. The The Boston Mountains are separated from the Ozark Plateau formations have been contributed by E. O. Ulrich; those of the Carbon­ by a steep descent, generally ranging in height from 500 to iferous, except the Winslow formation, by Gr. H. Girty. rocks of the Arkansas Valley consist of 24,000 feet or more of 700 feet, to which the name Boston Mountains escarpment has adjoining part of Missouri White River and its tributaries Carthage, Raytown (near Kansas City), Sullivan (in Franklin been applied. This escarpment is highest in its middle por­ have cut a wide basin in the Springfield Plateau. Much of County), and near Rolla, Mo. One of the two younger intru­ tion and gradually falls off eastward and westward to the this basin is included in the Eureka Springs and Harrison sions is a granitic dike near the mouth of Spavinaw Creek, in borders of the area. If viewed at. a distance from the Ozark quadrangles. Many remnants of the Springfield Plateau northeastern Oklahoma; the other is a dike, or possibly a plug- Plateau, on the north, this escarpment presents a bold, even, occupy the tops of elevations that stand out upon the Salem like intrusion, of graphic granite or pegmatite, on the southern wall-like front and a level crest, but in fact it is very irreg­ Plateau at greater or less distances from the Eureka Springs border of Camden County, Mo. ular, the plateau being dissected by numerous and deep escarpment. The Cambrian and Ordovician rocks outcrop almost wholly ravines. (See Pis. I and II.) Moreover, long, narrow spurs The Salem Plateau occupies southeastern Missouri and a within the Salem Plateau. The oldest formations are exposed project from the mountain area, and numerous knobs and large part of north-central Arkansas. Its surface is not level on the flanks of the St. Francis Mountains and in general dip peaks that are isolated from the main upland mass conceal the but dips gently away from the main divide of the Ozark away from these mountains, their outer margins therefore being escarpment and make the boundary between the mountains region and is deeply cut by stream erosion except in its central buried by later rocks. Those that form the floor of this plateau and the plateau transitional rather than abrupt, in spite of the part, where the valleys are shallow and troughlike, but the consist of a great mass of dolomites and limestones and of difference in altitude and of the local steepness of the escarp­ tops of the higher ridges and hills along its low dissected smaller amounts of interbedded sandstones, which belong in ment.