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The purpose of this study was to investigate in detail the core area of the Ouachita Mountains of southwestern Arkansas. As work progressed it was discovered that High Peak and Wheeler Mountain ridges are not homoclinal, as originally mapped by Miser and Purdue, but rather are anticlinal. According to our interpretation there are only two outcrop areas of Collier shale in the Crystal Mountains: those along upper Collier Creek and upper Huddleston Creek. This fact is stratigraphically significant in that almost no dark bluish gray limestone was found by the writers in these remaining two outcrop areas of Collier shale. Another significant change in the stratigraphy is that of the thickness and lithologic character of the Crystal Mountain sandstone, which Miser and Purdue d scribed as "almost entirely" sandstone and as being 850 feet thick. Pitt discovered that the measurable interval of the Crystal Mountain sandstone in the Crystal Mountains is only 180-320+feet. Lithologically the measured interval consists of two massive sandstone beds, 30-80 feet in thickness, separated by about 100 feet of thin beds of siltstone and shale. Locally as much as 50 feet or possibly slightly more of interbedded sandstone and shale makes up the uppermost member in the Crystal Mountains. In the Mount Ida area north of the Crystal Mountains the Crystal Mountain sandstone ranges in thickness from 280 to 370 feet because the uppermost member is thicker than it is in the Crystal Mountains. No conglomerate or other suggestion of an unconformity was found at the base of the Crystal Mountain sandstone; the writers therefore believe that the Collier shale is probably Ordovician in age rather than Cambrian.
The outcrop of supposed Collier shale north of Wheeler Mountain and near the town of Mount Ida is actually Mazarn shale. This Mount Ida area is one of sharp anticlinal folds with gentle westward plunge. Most of the Crystal Mountain sandstone ridges have been overturned southward. Faulting is rare throughout the Mount Ida area. The few observed faults are tear faults, but their presence suggests that minor thrust faults also may be present.
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