The Watercress Darter National Wildlife refuge sits in Jones Valley, the long, low-elevation area to the north of Red Mountain. The valley’s bedrock was formed back in the Cambrian Period (542–488 million years ago). Most of this rock is limestone or dolostone, but numerous hills within the valley are composed of chert. Learn more about how Jones Valley formed here.
Thomas Spring is the main geologic feature on the refuge. It emerges from rock of the Conasauga Formation. This is primarily a bluish-gray limestone with intervening layers of dark-gray shale. These rocks formed 501–513 million years ago, in the mid-Cambrian. At this time, most of Alabama was submerged in the tropical waters of the southern hemisphere. You can read more about the Conasauga Formation in the geology summary for the McWane Science Center page on Trek Birmingham.
This limestone is ancient, but it won’t last forever. Limestone and dolostone are alkaline rocks that slowly dissolve when exposed to acidic water. Rainwater is slightly acidic, and as it percolates through the ground, it becomes even more acidic as it absorbs acids released from the decay of plant material. These acidic waters gradually dissolve layer after layer of bedrock. Underground caverns systems are then formed over long periods of time. Caverns often host underground streams that flow beneath our feet as we go about our business on the surface. Occasionally, the roof of a cavern collapses and creates a sinkhole at the surface. At other locations, the underground streams can be forced to the surface to create a spring. The springs, sinks, and caverns of the region create unique ecosystems and support numerous plants and animals that add to the region’s impressive roster of species.
-R. Scot Duncan