Avondale Park is another Trek Birmingham site where geologic forces have created a place of great beauty and significance to our city. Visitors to the park will quickly notice the lowland section, which is part of the surrounding valley, and the steep hill that rises above. The valley floor is part of the Ketona Dolomite geologic formation, a thick band of carbonaceous (carbon-containing) rock that formed 501–513 million years ago in the mid-Cambrian Period. These are some of the oldest rocks at the surface in Alabama. Long-term exposure of this rock to the elements has created the flat floor of Jones Valley, where Birmingham is centered.
As an alkaline (pH > 7) rock, dolomite (and also limestone) is slowly dissolved by groundwater, which is slightly acidic (pH < 7). The underground streams that form as a result sometimes bubble up to the surface. King Spring, the centerpiece of Avondale Park, is one such spring. Roebuck Spring also bubbles up from the Ketona Dolomite formation 5.6 miles away to the northeast. It’s also worth noting that the Keytona Dolomite is a particularly pure form of dolomite and has been highly valued as a flux material in the making of pig iron and steel in Birmingham. It is still mined from several quarries in the city.
The hill from whose base the spring emerges is not part of Red Mountain, as some believe. It is an exposed chunk of the Copper Ridge Dolomite formation, which formed in the Late Cambrian Period, some 501–488 million years ago. This rock layer is composed of dolomite and chert. Rocks of the Copper Ridge Dolomite formation are more resistant to erosion than the other rock formations exposed on the floor of Jones Valley. Thus, most of the hills in the valley are part of this formation. The large gray rocks exposed where King Spring emerges from the base of the hill are dolomite. Pieces of chert can be found on the hillside – you can recognize the chert by its angular edges and medley of orange, white, and gray. Fossil stromatolites are common in the chert. These were colonies of microorganisms and the minerals they deposited when this part of Alabama was ancient seashore. Look for narrow bands of different colors in the chert that indicate the microbial layers.
The waters of King Spring have carved (dissolved, really) a cavern through the dolomite of the Copper Ridge and Ketona rock formations. Adventurous visitors to the park early last century would explore the cavern through which the spring flowed. The cave entrance was closed off in the 1970s and is no longer accessible. A description published in the Birmingham News said that a narrow passage went back 25-30 feet and then opened up into a large cavern.
-R. Scot Duncan