Rocks of Pennsylvanian age in the Ozark Highlands and Arkansas Valley are essentially the same as the sequence representing earlier Pennsylvanian deposition in eastern Oklahoma. Outcropping in the Arkansas Valley are the Atoka, Hartshorne, McAlester, Savanna, and Boggy formations; in the Ozark Highlands only the Morrow group and the Atoka formation are exposed. There is in general a gradual thickening of beds toward the south from the outcrop toward the Arkansas Valley, an eastward extension of the McAlester basin. A marked eastward thickening in the lower Morrow and Mississippian has recently been demonstrated by Maher and Lantz who tentatively assign these thickened units to the Jackfork sandstone and Stanley shale, respectively. This thickening apparently begins along north-south line through Pope County, which also approximates the eastern limit of outcrop of post-Atoka Pennsylvanian rocks. The significance of this facies change in terms of petroleum possibilities can not yet be determined. The Arkansas Valley, bounded on the south by the Ouachita Mountains and on the north by the Boston Mountains of the Ozark Highlands, is characterized structurally by long east-west-trending anticlines and synclines, many of which are faulted parallel with the axis. This folding had its beginning as early as Atoka time but the folding and faulting were not completed until after Boggy time. Along part of its northern margin the Arkansas Valley is separated from the Boston Mountains by a system of normal faults with a total vertical displacement as much as 3,000 fee . Pennsylvanian rocks in the Boston Mountains show a moderate regional dip south. Local folding is not as sharp and is not as common as in the Arkansas Valley. Much of the folding in the area had its inception in pre-Pennsylvanian time but almost all of the recognized faulting is post-Atoka. The anticlines of the Arkansas Valley have long been prospected for petroleum and to a slightly greater extent than those of the Ozark Highlands. The recorded production

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of the Valley for the past fifty-three years totals about ¼ billion cubic feet, essentially all from Atoka sandstones. It has been estimated that the present proved reserves amount to about four-fifths this amount. Much of the Arkansas Valley and most of the Ozark Highland area have not yet been adequately tested. During the past year an 8-million-cubic-foot well was brought in at 4,000 feet to establish the easternmost commercial production. It is in Pope County, half way between the Oklahoma line and the western margin of the Mississippi River Embayment.

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