Most of Petit Jean Mountain, one of several flat-topped ridges in the Arkansas River Valley, is in the southwest section of Conway County and consists of three distinct parts, known locally as Petit Jean, Rose Creek, and Cove Mountains (Fig. 1). Considered together, these parts almost encircle Ada Valley, which is about eleven miles in width. Elevations range from 1,120 feet above sea level at High Point to 323 feet at Ada in the valley. The local “Petit Jean Mountain” is the part on which Petit Jean State Park is located and on which Winthrop Rockefeller and many other families now live. Many interesting geological features are found in this section of the mountain. The most spectacular is Cedar Falls, where Cedar Creek plunges over a precipice almost 100 feet high, then flows on down through a rapidly-deepening canyon to Petit Jean River. Other interesting features are the Rock House, a large circular chamber about 120 feet in diameter and with a ceiling about forty feet high, the Palisades, the carpet and turtle rocks, Bear Cave Rocks, the Grotto, and the Natural Bridge, about seventy feet high with an open arch of about forty feet. These rock formations are in the hard, massive layers of the Hartshorne sandstone which overlays conformably the sandstones and shales of the Atoka formation of the lower slopes and valleys. The Atoka sandstones are slightly calcareous in places, but no limestone occurs. Fossils are found more frequently in the Atoka sandstones than in the harder Hartshorne. These are Paleozoic rocks of Carboniferous age. Petit Jean Mountain structurally consists of the Pontoon and Poteau synclines within the synclinorium of the Arkansas River Valley.