Like much of downtown Birmingham, the McWane Science Center rests on a foundation of rock known as the Conasauga Formation. This formation is found in Alabama, Georgia, and Tennessee and includes layers of limestone, dolostone, and shale. These rocks are incomprehensibly old, having formed 501–513 million years ago in the mid-Cambrian period of Earth’s history. At this time, most of that portion of Earth’s crust we know as Alabama was in the tropical waters of the southern hemisphere. It took 500 million years of continental drift for Alabama to reach its present location!
In the Birmingham region, the formation is mainly a bluish-gray limestone with intervening layers of dark-gray shale. The shale is derived from seafloor muds. In other areas of Alabama, the Conasauga Shale has been tapped for natural gas extraction.
The limestone is composed of calcium carbonate, the same mineral comprising chalk. It formed from the accumulation of hard body parts of marine reef–building organisms. Among them were corals and marine arthropods known as Trilobites. Trilobites were common in these tropical waters, and their fossils are abundant in some portions of the Conasauga formation.
Much of the heavy lifting of reef formation was done by green algae that used calcium carbonate to armor themselves against predators and the pounding of waves. When algae died, the calcium carbonate would settle to the seafloor, and the reef deposit would thicken.
Tiny single-celled Cyanobacteria helped build reefs in the Cambrian, too. These Cyanobacteria grew in colonies along the seashore, where they captured sunlight for photosynthesis. Lime-rich sediment trapped by the colonies formed mounds called Stromatolites. Stromatolites were once common in the world’s oceans but are now rare. The nearest active Stromatolites are in the Bahamas. In Birmingham, fossil Stromatolites are common in the Conasauga and other rock formations.
Altogether, the reef animals, the algae, and the Cyanobacteria built up massive reef deposits in the Cambrian oceans that became the Conasauga and other rock formations. In the Birmingham region, the formation’s limestone is described as argillaceous. This term refers to the high portion of clay particles in the rock. The clay originated with the weathering of rock on nearby land. Erosion of weathered rock was rampant in the Cambrian because there were no land plants to hold sediments in place with their roots. With every rain, rivers would wash great volumes of clay into the ocean. These clays bonded with the calcium carbonate sediments left by dead reef-building organisms.
The bottom floor of the McWane Science Center is below ground and embedded in the Conasauga’s limestone. The bottom floor features several aquaria with reef creatures from around the world. New reefs are surrounded by the old.
Sources: Lacefield, Geologic Map of Alabama (1989)