To many, geology may just be a bunch of rocks. But the geology of our land influences everything from the industry of our region to where our homes are built to the ecosystems all around us. All of these things are shaped by millions of years of geological processes: the collision of continents, the formation of mountains, subsequent erosion, drastic shifts in sea level, and the thawing after the ice age.
Birmingham, more than most cities, is founded on rock. Its rapid growth stemmed from the earth below it, which held iron ore, coal, and limestone – all the key components needed to make iron and create an industrial power.
The region’s complex geology has also shaped the ecosystems of the area. As different kinds of rocks weather, they produce different soils, which support different types of plants and different animals that depend on them. For example, the limestone bedrock at Ruffner Mountain Nature Center creates calcium-rich, low-acid soils that host plants like American Basswood, Slippery Elm, and Shumard Oak, while the sandstone glades at Moss Rock Preserve are home to unusual species like Nuttall’s Rayless Goldenrod, Pineweed, and Prickly Pear Cactus. The sheer variety of rocks crammed together in our area creates many options for ecological diversity.
But where do these different rocks come from? In Birmingham, all are sedimentary rocks formed hundreds of millions ago. Rivers carried sediments to seas, and the weight of those seas compressed those sediments and altered their chemistry, with the result being new rock. Beach sand became sandstone, mud became shale, reefs became limestone, and swamp muck with its thick deposits of dead plants became coal. In fact, if you look closely, you can find the fossils of swamp plants and tiny sea creatures in rocks all around the metro area.
Then, about 300 million years ago during the Pennsylvanian geologic period, the continents that later became North America, South America, and Africa began a slow-motion collision. Over millions of years, the earth’s crust buckled, snapped, and thrust upward, thereby creating the Southern Appalachian Mountains. The last phase of mountain building ended 250 million years ago, and ever since, the mountains have been worn down by wind, rain, and gravity to become the low mountains we know today. Those two steps – first the upward thrust, then the downward wear — exposed the sedimentary rocks, which provide many of the minerals we extract through mining to support industry in the region.
In much the same way that biologists have created ecoregions to help them map ecosystems, scientists who work with rock have created physiography, which divides the world into divisions, provinces, sections, districts and subdistricts depending on geology.
Parts of five physiographic provinces lie in Alabama. Two of the state’s three mountainous provinces intersect in the center of Jefferson County: the Valley and Ridge and the Appalachian Plateaus (some might know the latter as the Cumberland Plateau). Although the two provinces date back to the same series of events, they have different histories. Read more about Alabama’s physiographic sections here.
The Valley and Ridge Physiographic Province is known for its elongated ridges and intervening valleys, which run along a southwest-to-northeast orientation from central Alabama to New York. This convoluted topography developed when stress from the inter-continental collision caused the earth’s crust to deform. Throughout much of the Ridge and Valley, the continental crust was squeezed into upward and downward folds. In other areas massive sections of crust broke from the pressure and – through a process known as thrust-faulting – were pushed up and away from the collision zone. The mountains created through both processes were once steep and tall, but millions of years of erosion have worn them down to narrow ridges of durable rock that are the area’s mountains. Sandstone, which is exceptionally strong, is usually the rock found on the tops of the region’s mountains. It protects softer rocks beneath from erosion. The adjacent valleys are where the sandstone was fractured by mountain-building processes. The broken sandstone and much of the softer rocks beneath have eroded away.
Here in Birmingham, the most famous ridge is Red Mountain, a long linear ridge stretching many miles through north central Alabama. The corresponding valley, Jones Valley, contains most of Birmingham. The pattern is echoed in other ridges across the region: Shades Mountain, Oak Mountain, Double Oak Mountain, plus several smaller ridges crossing the metro area. Red Mountain is different from these other mountains in several ways. Most notably, it contains layers of hematite, or iron ore, and it lacks a layer of sandstone at its very top.
The Appalachian Plateaus Physiographic Province is found in the northern tier of the Birmingham metro area. In the portion of it found here, the Cumberland Plateau Section, areas of the up-thrusted crust have eroded in a different way, creating a network of broad, level mountains (e.g., Sand Mountain) with hard sandstone on top and softer sediments like limestone in the valleys below. Around Birmingham, the sandstone cap was thinner and most of it has eroded away. The softer sediments beneath are now eroding quickly in terms of geologic time. This has created the shale hills of northern Birmingham and beyond. Below those hills lie rich coal beds that are mined in the Warrior Basin.