Keel Mountain Geology
By Bob Baudendistel
First, let me start with the disclaimer that I am not a geologist despite my always having a curiosity for it. My interest with the geology of Keel Mountain quickly followed my initial purchase of land atop it back in 1987. The resources and materials which are required for studying the geology and make-up of this mountain, along with the rest of the Cumberland Plateau, are all readily available. The Cumberland Plateau was formed when the continental plates collided. The resulting “folds” and outer ripple effects are what then shaped the Cumberland Plateau and much of the Tennessee Valley.
Keel Mountain is essentially composed of about 8 separate layers of sedimentary rock. These include various types of limestone and two distinct sandstones. The top of the mountain has what is known as the “Pottsville” sandstone. Its thickness averages about 85 feet with some even thicker seams close to the Madison/Jackson County line. This rock layer is highly porous meaning that water travels through it quickly. At the base of this sandstone is a thin seam of coal which was often mined. Many natural voids and other fractures in the sandstone cap rock provided the perfect means for the miners to access the coal. These mines and other access points are often found along the base of the towering sandstone bluffs which encompass much of the mountain. Sedimentary sandstones typically begin with the mechanical and chemical erosion of quartz and silica from other places. Natural bonding agents including iron are often mixed in with the quartz and silica making up the sandstone rock. The high iron content is most noticeable where the numerous intermittent springs usher water along the lower edges of the sandstone cap rock and often leave a reddish tint in the channels.
For more information see Keel Mountain Geology Spreadsheet
Continuing below the sandstone is a seam known as the “Pennington” formation. The Pennington formation is unique in that it contains many more shale fragments than the remaining limestone types found deeper in the mountain. Evidence of this rock layer is most noticeable if one were to be out hiking the trails which are found directly below the towering sandstone cliffs found atop the mountain. While standing upon this layer, a moderate slope is found running outward towards the valleys. Large waterfalls are another key feature to look for in the hollows where this rock becomes even more weathered and exposed.
Bangor limestone is the thickest layer found in the geologic cross section of the mountain. Its thickness ranges anywhere from 300-400 feet. The best place to actually see this particular rock is while driving over towards Huntsville. The freshest and cleanest view of the rock is along the newer Cecil Ashburn Drive just as it tops the ridge. The rock can also be found exposed along the uppermost peak of the U.S. Highway 431 corridor while heading into Huntsville. All limestone rock layers have unique qualities, but they all share part of the same sedimentary origin where calcium from sea shells and other aquatic life once settled to the floor of oceans. The calcium itself is most pure when it is found in the form of calcium carbonate.
Bangor Limestone is unique when compared to other limestone rocks because it is highly dolomitic. Dolomite gives this rock a slightly darker gray appearance and helps make it more crystalline. Both the impact and compressive strength of this particular limestone helped make it the preferred locally abundant rock for use in the earliest bridge piers, retaining walls, and other structurally significant portions of the infrastructure prior to the use of reinforced concrete.
Hartselle Sandstone is the most sparsely found rock throughout the geologic map of the Keel Mountain. One of the most noticeable exposures of this rock occurs back along McMullen Road at what historians like to refer to as “Candlestand”. This sandstone differs in many ways from the Pottsville Formation found atop the mountain. The grains of sand which make up the Hartselle Sandstone tend to be thicker and carry more of a “chiseled” shape. Surface exposures of this sandstone tend to be very rare across the layout of this particular mountain because it is often covered over by a thicker overburden. The best evidence of the rock will occur whenever the mountain and its topography feature a more noticeable mid-level projection out away from the normal slope of the mountain. Like the Pottsville formation, this sandstone is highly porous which aids with the forming of many intermittent (wet weather) springs ushering their water from the remaining limestone layers found below.
Next, we find the Gasper Limestone. This is one of my favorite rock layers in that it carries the unique adjective “argillaceous” in its description. The word itself simply means “clay containing”. While out in the field exploring the mountain, its trail systems, and creek hollows; I noticed that this particular rock layer tends to be the most cavernous given the countless number of cave entrances that occur within this rock layer. So much so, that special infrared cameras and other remote sensing tools often show higher concentrations of heat signatures stemming the topographic contour intervals where this rock gets exposed. One theory behind this is that the higher clay content of the rock, hence the descriptor “argillaceous”, would suggest that the rock has the ability to retain water for longer periods. Given that the majority of limestone caves are formed by the slow dissolution of limestone from the water and its acids, then this would lead one to conclude that the caves would in fact be more numerous. Perennial springs which are most frequent throughout the lower 1/3 of the mountain further compliment this same theory.
Whenever a limestone quarry is operated, the Gasper Formation is one of the most preferred rocks when used for construction purposes due to its higher clay content. The manufacture and grading of cement powder that gets included with concrete is greatly affected by the quality of the limestone that is used, including its clay content. Roadways also benefit from the use of this particular rock when it is crushed and used for the proper buildup of a sub-base layer.
The St. Genevieve Limestone is very similar to the Gasper. In fact, many geology reports even identify these two together as the very same layer. Still, many reports suggest that the St. Genevieve limestone can be distinguished by its lighter gray appearance. Like Gasper, it too is abundantly fossiliferous. When labeled as a separate layer, the St. Genevieve limestone is most often the initial layer of limestone encountered while ascending up the slope of the mountain from the reaches of any surrounding lower valley.
Throughout the valleys which encompass the mountain, much of the floor is underlined by the Tuscumbian Limestone. Some outcroppings can also occur along the lower edges of the mountain. This limestone is very similar in chemical composition to the St. Genevieve and Gasper layers often found above, but it carries the unique identifier of being more crystalline and having more imbedded chert layers. Both the Paint Rock and Flint River Valleys that are seemingly split by the mountain feature numerous outcroppings and exposures of the Tuscumbian Limestone. The red color residuum clays that dominate much of the local landscape mostly stem from the chemical weathering of Tuscumbian Limestone. Civil engineers often deal with this particular limestone and its resulting overburden clays whenever working with design of larger building foundations and bridge piers.
While the weathering of this particular rock will create the red clays we often see, a lot of the acreage that surrounds the City of Gurley features another type of soil. While driving west from downtown along Little Cove Road, one only has to go a relatively short distance before the land suddenly becomes subject to the inundation of flood waters. Areas which are prone to flooding often feature soils that are high in organic matter and finer silts. This explains the reasoning behind the removal of the topsoil and replacing it with a different type of clay wherever a new building is going in. The native soils found within the flood areas simply could not support the load of a building, for instance, if things were left alone. The geology of Keel Mountain is fairly consistent with that which is found throughout much of the Cumberland Plateau. Still, it remains highly unique to our local area in that it lies along a topographic breakline separating the Paint Rock and Flint River valleys. If a core sample were taken from within the floodplains of each river, the results would greatly differ. For this reason, anyone who studies the geologic profile of Keel Mountain and the surrounding areas should not assume that what is found in one particular place will likely be the same somewhere else.
After I became interested in the geology of Keel Mountain and began researching it, I immediately viewed everything about the mountain much differently whenever I was out hiking its trails, gathering fruits, bicycling, canoeing along the rivers, meeting with its people, or admiring the view from one of its bluffs. Having an understanding of the geology gives one an enhanced sense of commitment to the mountain, its people, and its preservation for future generations.
Malmberg, Glenn T. and H.T. Downing. Geology and Ground-water Resources of Madison County, Alabama. County Report 3. University of Alabama Press. 1957.
(note: this and many other geology books are available at the Huntsville – Madison County Public Library in the Heritage Room section.)
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Web page created December 14 2009.