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Earth Geology Rock Structures Ozark Dome

The Ozark Dome

The entire Ozark area is a structural dome. A dome is a structure in which the center is high and the rock layers slope or dip away from the center in all directions. The Ozark dome is asymmetric (uneven) with steeper dipping rock layers on the east flank. 

The center of the dome is the area around the St. Francis Mountains in eastern Missouri. The oldest rocks, Precambrian granites and volcanics, are found in this central area. The dome is an eroded dome, so as you travel away from the central area you encounter rocks of increasingly younger age. The slope or dip of the rock layers is very gradual away from the center, such that in an individual outcrop the layers appear horizontal. 

Within the Bryant Creek watershed this regional dip is to the south and west. Perhaps the most observable is the Roubidoux sandstone layers, which are so prominent over most of the watershed. The Roubidoux plunges below river level near Tecumseh and is not present to the south even though the river valley elevation is lower. The Roubidoux is also absent from the river valleys to the west of the Bryant, except for a limited outcrop along Cowskin Creek west of Ava. 

The base of the Roubidoux is at an elevation of approximately 1,000 feet in southern Wright County along the northern edge of the watershed, and at an elevation of about 500 feet near Tecumseh at the south edge of the watershed. This is a drop of about 500 feet in a distance of 25 miles, which corresponds to an average slope or dip of 20 ft/mi. Part of this elevation difference is undoubtedly due to faulting along the Mansfield Fault system, which parallels the Bryant watershed.