The landscape of Jemison Park is the result of millions of years of geological activity, but many of the landforms we see in and around the park are of relatively recent origin. About 260 million years ago, when the mountain-building forces that created the Appalachian Mountain range on the eastern side of North America began to subside, the land that is now Jefferson and Shelby Counties formed the southern part of that range. The Appalachians rose to about 5 miles above sea level, about the height of the present-day Himalayas, far higher than the eroded remnants we see today. Over time, erosion reduced these mountains to the landforms that comprise Alabama’s Valley and Ridge physiographic section. The primary agent of this erosion was water, principally in the form of rainfall. Rain continues its work today, and, although the pace is very slow, changes are visible and ongoing throughout Jemison Park.
In the park, Shades Creek meanders through Shades Valley, elevation about 600 feet above mean sea level (msl), which is bordered by Shades Mountain, 1,050 feet msl to the southeast, and Red Mountain, 900 feet msl to the northwest. These two mountains, as well as Oak Mountain and other nearby ridges, are remnants of the Appalachian Mountains, whose countless tons of eroded rock were washed downstream into Mobile Bay and the Gulf of Mexico.
The northwest face of Shades Mountain, a precipitous cliff in many places, reveals some of the history of the area. Like many mountains, Shades Mountain and Red Mountain were built by layer after layer of rock being laid down in seas, salt and fresh water, over many millions of years and later raised high above sea level by movements in the earth’s crust. Some of these layers are exposed in the cliff face of Shades Mountain at Jemison Park. On page 15 is a stratigraphic column showing the sequence of rock layers in Jemison Park and the surrounding area. This column was prepared for this booklet by the Geological Survey of Alabama, located in Tuscaloosa. The portion of the column between “Shades Mountain” and “Shades Creek” shows the layers that are visible in the northwest face of Shades Mountain. The column also shows the layers that make up Red Mountain. The level of the streambed of Shades Creek is also shown. The cross-section on page 14 shows the present configuration of these rock layers, which are tilted – or dip – toward the southeast. This illustration is based on a diagram prepared by Dr. Denny Bearce for a field trip through the Red Mountain cut and is reprinted with his permission.
Very generally, all of the layers that are exposed on the northwest face of Shades Mountain also rested in the same order upon the present southeastern slope of Red Mountain but were subsequently eroded away. Shades Creek was the primary agent of this erosion, along with the southwestern drainage between the ridges of Red Mountain and Shades Mountain. Shades Creek acted not only as an erosive agent, by removing the north face of Shades Mountain, but also as a builder of the flood plain by depositing some of the material upon which Jemison Park is located.
Today, Shades Creek at the Cahaba Road bridge drains 15.1 square miles of land surface. Because much of that surface is now paved, for example the Eastwood Mall area, rainfall flows quickly downhill as runoff. When the ground becomes saturated after heavy rains, additional rainfall can result in flooding, which we see from time to time along Mountain Brook Parkway.
The flood plain, best seen on the creek side of Mountain Brook Parkway between Cahaba Road and Overbrook Road, began its most recent growth phase just a few thousand years ago when the last Ice Age ended and meltwater from the receding glaciers rushed seaward, reshaping the landscape. The power of the flooding can be imagined by looking at the rock outcroppings on the northwest side of the creek at the second bridge after Cahaba Road (the first bridge spans Watkins Brook). More dramatic still are the stone cliff faces bordering the upper reaches of Watkins Brook at Memory Lane where Santa Claus used to sit at Christmas. The white and gray rock exposed here is Hartselle Sandstone, a very hard rock consisting mostly of quartz that was deposited about 330 million years ago.
Some of the rock layers in and around the park are more permeable than others, which means that rainwater seeping into the ground can flow through them more easily. When a fracture or human excavation breaks into these layers, springs may be created. One such spring, which flows during wet weather, can be seen on the east side of Watkins Brook about 100 yards upstream from Mountain Brook Parkway.
Today, the flood plain is still changing. The stream bed of Shades Creek is composed of a variety of materials including bedrock, loose cobbles and boulders, and sand and gravel. These materials are removed, transported, and deposited along the dynamic interface between rushing water and stream bed. Trees in the flood plain are sometimes uprooted by floodwaters, and when this happens near the creek the bank is torn apart there and the stream bed begins to widen.
The large sand and gravel bar downstream from the Beechwood Road bridge changes with each flood, generally growing from the Nature Trail side of the creek towards Woodhill Road. As the bar grows, the channel becomes narrower and forces the water to flow more swiftly, eroding the bank on the Woodhill side and eventually altering the flood plain and the path of the creek itself.
Another recent change in the flood plain is the island in Shades creek below the Old Mill just before the creek passes under the bridge. Older maps, as well as the recollections of some of the park’s patrons, including Bill McDonald, confirm that the island did not exist until 1989, when a new channel was formed closer to Mountain Brook Parkway than the main channel, creating the island.
Flood plain soil comes not only from the rock tumbling down Shades Mountain but also from rock derived from the southeastern slope of Red Mountain and from upstream to the stream’s source in Irondale. The dominant soil in the flood plain is called the Sullivan-State complex. This soil is very sandy and quite permeable and is best suited to water-tolerant flora. The soil along the Nature Trail above the flood plain is called the Leesburg-Rock Outcrop complex and is host to a wide variety of plants. The richness and diversity of life is evident throughout the park as the creek meanders along the flood plain, subtly reshaping the land by its passage, helping one seed, hindering another root.
We are deeply indebted to the Geological Survey of Alabama for its help in the preparation of this booklet.
– Thomas N. Carruthers