OKLAHOMA

synclinal trough, the sedimentary beds of which are thickest in the center and grow thinner toward the north and south. There are a great many local east-west rock folds in this area which were formed by the uplift of the Ouachita region during the late Pennsylvania period. See map. The Arkansas Valley region is made up largely of shales and sandstones of Pennsylvania age, which, in the eastern portion, have been estimated to have a thickness in excess of 25,000 feet. In the western portion these shales and sandstones contain the commercial coal beds of the State. Passing southward from the lower portion of the Arkansas Valley, these Pennsylvanian rocks grade into Mississippian shales and sandstones lying on the north flank of the Ouachita Uplift.

The Ouachita Mountain region consists of a well defined uplift of great size with a more or less central east-west axis. The whole

region has been subjected to intense lateral folding. The rocks of the Ouachita Mountain region consist principally of shales, slates, sandstones, and cherts, with a relatively small amount of limestone and novaculite. The oldest rocks exposed in the Ouachita Mountain Uplift lie along the central portion of the great Ouachita fold and, in general, decrease in age both north and south of the central axis of this fold. The oldest beds in part of the central portion of this region are of Cambrian age and, passing either north or south of the central axis of the fold, include the rocks of middle and upper Ordovician, Silurian, and Carboniferous age. See map.

The Athens Plateau makes up the south flank of the Ouachita Uplift. There are some local folds and faults in this area, but they are not as great in number as in the region immediately north of

LOUISIANA

the Ouachita Uplift. The rocks of the plateau are Pennsylvanian and Mississippian in age and have a small regional dip to the south.

The northern and eastern portion of the Coastal Plain of Arkansas makes up the western half of a great embayment or synclinal trough extending northward into central United States. See map. The beds in this portion of the Coastal Plain dip gently to the southeast toward the axis of the embayment, which approximates the position of the Mississippi River. The southern portion of the Coastal Plain of Arkansas is not so much influenced by the presence of this synclinal, trough and the beds dip more to the south than to the southeast.

The rocks of the Coastal Plain series in northeastern Arkansas are deposited over the eastward extension of dolomites and limestones of the Salem Plateau. In eastern Arkansas, south of approximately the north line of Cross County they are deposited over what is probably a fairly level plain made up of shales and sandstones which are an eastward extension of the Arkansas Valley region. In the central and eastern portions of the Coastal Plain they are deposited over the steeply inclined Ouachita Mountain series of rocks, and in the southern and southeastern portions of the Coastal Plain they are deposited, so far as is known, over Carboniferous rocks which have probably been eroded to a fairly level plain. These Coastal Plain formations feather out along the border of the Coastal Plain beds and thicken to over 4,100 feet along the southern border of the State, and probably to over 5,000 feet along the southeastern border, and are about 2,200 feet thick along the northeastern border of Mississippi County.

GEOLOGICAL MAPPING

The portions of Arkansas which have been geologically surveyed are shown on the accompanying index map.

To show the portions of the State which have been mapped, the area of the State has been divided into three parts: (1) The portion which has been completely surveyed is shown by the cross hatching and constitutes 11.2 per cent of the area of the State, or 5,850 square miles. In this area practically all of the geologic formations have been mapped and classified. (2) The portion which has been partially surveyed is shown by the single hatching and constitutes 65.3 per cent of the area of the State, or 34,308 square miles. This area is only partially surveyed because some of the formation groups have not been broken up or classified and the structural features of the area have not been fully worked out. (3) The area which is unsurveyed and is left blank on the map constitutes 23.5 per cent of the total area of the State, or 12,367 square miles. This area has not been geologically surveyed as no detailed reports have been made of this region and the geologic formations. and structural features are known only in a general way.

The accompanying list shows the principal reports which contain geological maps of the State. Their numbers correspond with those shown on the index map.

INDEX TO GEOLOGICAL MAPS

1. U. S. Geological Survey Professional Paper, No. 46, "Geology and Underground Water Resources of Northern Louisiana and Southern Arkansas," by A. C. Veatch. (Out of print.)*

2. U. S. Geological Survey Water Supply Paper No. 399, "Geology and Ground Waters of Northeastern Arkansas," by L. W. Stephenson and A. F. Crider. (Out of print.)*

3. Arkansas Geological Survey Annual Report for 1890, Vol. II, "Igneous Rocks," by J. Francis Williams. (Out of print.)

4. Arkansas Geological Survey Annual Report for 1890, Vol. III, "Novaculites," by L. S. Griswald. (Out of print.)*

5. U. S. Geological Survey Folio, No. 215, "Hot Springs Geologic Folio." (Obtainable from U. S. Geologic Survey, Washington, D. C., for 25c.)

6. Arkansas Geological Survey Report for 1909, "The Slates of Arkansas," by A. H. Purdue. (Obtainable from Arkansas Geological Survey, 446 State Capitol Building, Little Rock, Ark. For postage charges see table on page 271.