Oak Mountain State Park sprawls across the ridge of Double Oak Mountain, actually twin ridgelines, an unusual feature that probably inspired the mountain’s name. Between the two ridges is a shallow valley and a small stream, Peavine Branch, that begins its route down the mountain at Peavine Falls.
The mountain was formed from rocks of the Pottsville Formation some 300 million years ago during the Pennsylvanian geologic period, the same epoch that saw the birth of the Appalachian Mountains. During this time period, the African, North American and South American continental land masses slowly collided, pushing the earth’s crust upwards into jagged peaks. Over time, water and weather wore these dramatic mountains down to the narrow ridges that now line the horizon in our part of Alabama.
Double Oak Mountain’s twin ridgelines were formed when a long section of the crust was pushed from the southeast and stacked upon sections to the northwest – a process known to geologists as thrust-faulting. The line where this occurred is called the Shackleford Gap Fault, and the Peavine Branch follows its path. Each ridge and the mountain’s southeastern slope is made up of the same rock layer, Pine Sandstone, that was separated by the fault.
This sandstone, despite its name, is sturdy stuff. The mountain’s northwestern flanks, however, are made up of other rocks that are more susceptible to weathering and age. On the north side, the park’s hills are based on the Parkwood Formation, a set of shales and sandstones formed in the very early Pennsylvanian and very late Mississippian geologic era, 350 to 320 million years ago. These rocks have eroded away more quickly, resulting in lower hills.
-R. Scot Duncan