Geology of Red Mountain Cut

Before the ‘Cut’ was created to build an expressway linking downtown Birmingham and the neighborhoods south of Red Mountain, the mountain’s geological features remained hidden in the rock but slicing through the mountain changed this drastically. Now, 190 millions of years of geologic history can be appreciated and studied. In fact, the Red Mountain Cut exposes more geologic history than any road cut in the United States. Strata within the cut are inclined approximately 20 degrees toward the southeast, parallel to the general trend of the cut.

The upheavals of the previously horizontal rock formations, now visible in the Red Mountain Cut, are the result of this collision between the two continents. Not only does the rock contain deposits of raw material, including metallic ores, limestone, and coal, but clear distinctions between millions of years of geological rock layering are visible. Also exposed in the rock are a myriad of fossils, minerals, and rock types that tell Alabama’s geological history.

Once a dome-shaped “anticline” landform, the Red Mountain Cut now bears a resemblance to a steep valley-like formation. The Valley and Ridge province of the Appalachian Mountains, where the Red Mountain Cut is located, contains sedimentary bedrock that was deformed by folding and faulting nearly 200 million years ago. The bedrock was horizontally compressed, forming major anticline (dome-shaped) and syncline (bowl-shaped) folds. These folds were then broken by major fractures called thrust faults that caused portions of the folds to be displaced northwestward.

Geologic formations from five periods of the Paleozoic Era are exposed here:

  • Upper Cambrian Copper Ridge Dolomite
  • Middle and Upper Ordovician Chickamauga Limestone
  • Silurian Red Mountain Formation
  • Devonian Frog Mountain Sandstone
  • Lower Mississippian Maury Shale
  • and Middle Mississippian Fort Payne Chert, Tuscumbia Limestone, Hartselle Sandstone, Bangor Limestone, and Floyd Shale

To help remember the order of the rocks through the six periods of the Paleozoic era, try this mnemonic: Chris Ordered Some Delicious (mushroom and pepperoni) Pizza. From oldest to youngest, that’s Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. As you face the cut, the rock closest to downtown Birmingham is the oldest within the formation, and the rocks south toward Homewood are younger.

During the construction of an interpretive trail in the Cut, in 1971, metal arrows were placed in the rock to point out unique features to the visitors on the trail below. One arrow points to the “reef” of marine animals such as trilobites frozen in time in the Ordovician period. A layer of volcanic ash after the deposit of marine reef would have been from a volcanic eruption sometime around 450 million years ago. Although many of the layers have been eroded by water and wind, these sandwiched time capsules tell us roughly the order in which the rocks were laid down, like pages in a book. There is a large “unconformity” in the Cut, meaning there is a large chunk of missing time. In other words, there are a few missing pages from this book of geologic history.

The drawing below shows a “fault” where the northwest part of the mountain broke under pressure from below. To see where the strata layers were before the upward thrust, match the textures of the rock. Did you notice the “beach”? Up close, this looks like a sand sandwich! A layer of sand and gravel pressed together from weight on top and upward pressure from below is preserved just under a layer of Red Mountain iron ore.

RMC Graphic

Try your hand at finding the “lost years.” Print this sheet and fill in each layer with a different color to see how the mountain has changed over time.