The hilly, northern portions of The Gardens are underlain by the Hartselle Sandstone. The layer is formed of sand and other offshore sediments deposited when the region was a shallow sea in the Mississippian Period (318-359 million years ago). This durable rock is more resistive to weathering and erosion than the bedrock of adjacent areas. Soils weathered from sandstone tend to be acidic because typical sandstone doesn’t contain any minerals to neutralize acids created by the decay of plant parts on the forest floor. However, the original character of the soils at The Gardens has been heavily modified over the decades.
A quarry within The Gardens dates back to the 1930s when the Works Progress Administration extracted Hartselle Sandstone to build structures in Lane Park (the area now occupied by The Birmingham Zoo and The Birmingham Botanical Gardens) and to build homes in nearby Mountain Brook. Today, the quarry hosts the Kaul Wildflower Garden and a landscape reminiscent of Alabama’s montane ecosystems.
The lower regions of The Gardens are 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 also date back to the Mississippian Period but are slightly younger than the Hartselle Sandstone. The sediments that became this rock were originally deposited when the region was an estuary similar to Mobile Bay. In natural settings, shale is rarely seen at the surface because it quickly weathers into soil when it is near the surface. Original surface soils over these shale layers were probably acidic or possibly circumneutral if the shale’s limestone was abundant.
Source: Pawlewicz, Mark, J., and Hatch, Joseph R., 2007, Petroleum assessment of the Chattanooga Shale/Floyd Shale–Paleozoic Total Petroleum System, Black Warrior Basin, Alabama and Mississippi, in Hatch, Joseph R., and Pawlewicz, Mark J., compilers, Geologic assessment of undiscovered oil and gas resources of the Black Warrior Basin Province, Alabama and Mississippi: U.S. Geological Survey Digital Data Series DDS–69–I, chap. 3, 23 p.