Map Courtesy of ODNR Geologic Survey
Rock formations from the Ordovician to the Mississippian age are present in the preserve system and EOA is possibly the only place in Ohio where you can see all of these formations in such a small area.
The lowest layers are of Ordovician age and contain Richmond limestone and shales. These are calcareous bedrocks and are found in the lowest areas in the preserve system along Ohio Brush Creek. This layer is mostly covered by alluvial soils. However, it is exposed along Beasley Fork just west of EOA.
The Silurian formation is the next rock formation above Ordovician. It consists of Brassfield Limestone, Estill Shale (Crab Orchard Shale in Durrell and Durrell 1975), and Lilley-Bisher and Peebles dolomites. Brassfield Limestone is the lowest layer, and forms rock terraces and small waterfalls. This layer is calcareous and about 15 m. (50 ft.) thick. The rock is a fossil rich, well-bedded crystalline limestone (Durrell and Durrell 1975) and was mined for its iron content in the early 1800s.
Above Brassfield Limestone is Estill Shale, which quickly disintegrates when exposed to water, forming a sticky clay-loam. This layer is mildly acidic and is about 40 m. (130 ft.) thick. Steep gullies quickly formed after the soils were exposed by settlers, and were abandoned once the soils were unproductive.
The next layer, Lilley-Bisher Dolomite, forms cliffs and waterfalls and is about 10-15 m. (40 ft.) thick. Lilley-Bisher is a combination of Bisher and Lilley dolomites and the presence and thickness of the two layers fluctuates in the preserve system. These formations are grouped together due to their very similar physical appearances. Bisher Dolomite is similar in texture to sandstone but contains primarily calcareous sands. It is light to medium gray in color and weathers to a brownish-orange color. Lilley Dolomite is a light to medium gray dolomite with fossil fragments (Swinford 1985).
Above Lilley-Bisher is Peebles or Cedarville Dolomite (Braun 1928a). This layer has an open, cavernous structure and is rather resistant to weathering. Because of its resistance to weathering, it forms the largest cliffs in the preserve system. The dolomite weathers differentially, forming a honeycomb appearance. The formation is about 20 m. (70 ft.) thick and contains very little iron, but does contain minerals such as pyrite or limonite, which gives the rocks a gray or whitish color (Durrell and Durrell 1975, Stout 1941).
Above the Peebles Dolomite is the Ohio Shale (Ohio Black Shale in Durrell and Durrell 1975) layer of the Devonian stratum. This layer is rather steep, acidic and is nearly 91 m. (300 ft.) thick. The last layer, Berea Sandstone of the Mississippian formation occurs only on the highest peaks in the preserve system. These bedrocks are acidic and are less than 30 m. (100 ft.) thick.