Geology of Sloss and the making of “The Magic City”

Today, Sloss seems a bit like a fish out of water, old blast furnaces sitting in a modern downtown. The location of the furnaces is due to geologic processes that began 300 million years ago and involved ancient oceans, swamp forests and violent geologic events.

Birmingham’s very existence is tied to the ancient marine and fresh waters that once covered Alabama. The city’s growth was based on the pig iron needed to make steel, foundry iron, and cast iron. Iron production requires three ingredients, all of which can be found within a stone’s throw of downtown.

First, there’s iron ore, or hematite, found in the rust-colored rock formations that give Red Mountain its name. Then, to melt the iron, manufacturers needed ready fuel to make intensely hot fires. Fortunately coal – which was converted into a cleaner, hotter-burning fuel called coke – is also abundant in the areas northwest of the city. Finally, the smelters needed a flux material that could be added to the furnaces’ hot fires to bind with the ore’s impurities and draw them from the molten iron. Limestone, the initial flux of choice, was readily available along the level floor of Jones Valley – which also provides the perfect flat spot to locate the rail lines needed to bring raw materials in from the surrounding hills and take pig iron back out.  Later, dolostone from the valley floor was quarried when its superiority as a flux was discovered.

These crucial elements are all here because of the way the rocks beneath Birmingham were formed. Limestone and hematite are the products of ancient seas that covered Alabama more than 400 million years ago, in the Silurian and Ordovician geologic periods. Limestone formed from the accumulated remains of hard-bodied creatures like corals that lived in shallow reefs. Hematite formed after iron-rich clays that eroded from nearby volcanic mountains were deposited on the sea floor; specialized bacteria may have helped concentrate the iron into hematite.

The coal formed later, roughly 300 million years ago during the Pennsylvanian geologic period, when parts of Alabama were covered with vast swamp forests much like those in the Mobile River Delta. There, dead leaves and stems accumulated in stagnant waters for millions of years. Later, they were buried beneath other sediments, compacted, and chemically transformed into coal.