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.