A hike through the preserve can take you across three different geologic layers that span more than 325 million years of earth’s history. The lower section of the preserve along the creek is made up of recently eroded material deposited by Shades Creek over the last few thousand years. Geologists call such material alluvium. It contains a mixture of different rock types, including chert, sandstone, and limestone, and different sediment sizes, including cobble, gravel, sand, and silt. The upper layers are mixed with debris from humans, including glass and bits of metal.
As you take the trail up into the preserve, the rocks deep below your feet are part of the Parkwood Formation, a sedimentary rock layer formed sometime in the interval between 328–311 million years ago (sedimentary rocks form from oceanic deposits). This was the Late Mississippian subperiod, a time when much of northern Alabama was covered with a shallow tropical ocean. At that time, the African and South American continents were drifting toward North America, and the consequent distortions of the earth’s crust created mountains rising in adjacent areas. As these mountains eroded, muds and sands were spilled into the nearby oceans. These materials settled on the bottom and, over the next few million years, became the Parkwood Formation. Muds became its shales, while sands became its sandstone. Limestone, a sedimentary rock formed of coral reef debris can also be found. Because reef-building organisms need a lot of light, the presence of limestone tells us that the Late Mississippian ocean covering Alabama was sometimes clear enough to allow reefs to flourish.
If you hike all the way to the top of the mountain, you’ll be standing above rocks of the Pottsville Formation. This thick layer of sandstone formed in the Early Pennsylvanian subperiod (318–312 million years ago), as the ocean covering northern Alabama shrank and the Southern Appalachian Mountains grew. The sand came from erosion of rocks freshly exposed in the nearby mountains, and it accumulated on the floor of the ocean basin for millions of years. The durable nature of the Pottsville Formation sandstones helps create Shades Mountain, on whose slopes the Homewood Forest Preserve is located. The cap of Pottsville sandstone protects the softer rocks of the Parkwood Formation from erosion. Without this cap, much of Shades Mountain would have eroded away over the ages. With most of Alabama covered with sedimentary rocks such as these, the ancient oceans of the past are present with us every day.
– R. Scot Duncan