Though you won’t see this rock at the surface, the Birmingham Zoo is underlain by a formation known as the Floyd Shale and Bangor Limestone Undifferentiated. This layer is predominantly shale with an intermingling of limestone and some mudstone. These rocks date back to the Mississippian Period (359-318 million years ago). The sediments that became these rocks were originally deposited in an estuary not unlike Mobile Bay. In natural settings, shale is rarely seen at the surface because it quickly weathers into soil deep below ground.
The hilly and undeveloped northern portion of the Zoo property is on the Hartselle Sandstone. This rock formed from sand and other near-shore oceanic sediments deposited during the Mississippian. Sandstone is more resistive to weathering and erosion than the shale and limestone beneath the Zoo’s exhibits – this is why these hills exist.