Lost Worlds of Alabama at Moss Rock Preserve

We are so glad you asked!  Here is a question about Moss Rock Preserve answered by Jim Lacefield – follow the page references in Lost Worlds in Alabama Rocks: A Guide to the State’s Ancient Life and Landscapes (2nd Edition) (2013).

One thing puzzles me. The Sandstone boulders at Moss Rock, how did they get there? Were they buried in the landscape and uncovered through the erosion of the valley since they were resistant to erosion? Or did they tumble off the mountainside and come to rest to then weather in place?

First of all, the sandstone boulders are from the lower part of the Pottsville Formation (probably the Shades Sandstone Member, named for exposures on Shades Mountain). These Pennsylvanian strata are discussed on page 132 in the new edition of Lost Worlds. They were deposited during an unusual stage of Earth history when sea levels took a precipitous drop due to the onset of an ice age centered on the south polar region. This ice age would eventually last more than 40 million years—well into the Permian Period. Coal Age Alabama was sitting in the southern tropics far from the glacial buildup, but the fall in sea level caused by this glaciation had major effects on the state’s shoreline and coastal plain. A drop of several hundred feet caused the shoreline to shift from somewhere not far southeast of present-day Birmingham off to the northwest across the Black Warrior Basin. This rapid sea level fall left the sandy beaches and offshore sand bars exposed to extreme erosion in the rainy, tropical climate. All of this sandy material was transported downhill by gravity toward the retreating shoreline. In many places such as at Moss Rock Preserve the sandstone is massive, with few indicators of current direction or wave action. Some geologists (Jack Pashin, for example) believe these massive, coarse-grained sandstones represent sand-filled river valleys carved into the exposed continental shelf that extended far off to the northwest (see maps on pages 133, 149). When the sea would return during the following interglacial stage shale would be deposited on top of these sandy strata and then back again as the next glacial cycle begins. You can see an exposure of Pottville shales and sandstones stacked together in an embankment in the WalMart parking lot in Vestavia Hills (page 138 in book).

Now, to the second part of your question: The Pottsville sandstones once extended as a single, continuous unit all the way northwest across the Black Warrior Basin (although not necessarily the same thickness or identical time of deposition). The folding of the Birmingham anticlinorium fractured the strata as the land was folded upward (pages 65 & 66), and all of the layers that once covered Jones Valley were eroded down to Cambrian strata (more than 3 miles of rock!), including the Pottsville Formation that once covered the uppermost part of the fold. The southern flank of the fold eroded back to the southeast, eventually to become Shades Mountain. The very resistant Pottsville sandstones held up to the millions of years of erosion better than the surrounding rocks and were left exposed as softer strata were removed over time. Cracks and joints in the sandstone gradually became larger and larger, eventually producing those huge boulders at Moss Rock. Gravity has transported some of the boulders downhill over great time, so they do not necessarily sit in-place. Lower Pottsville sandstones such as these create some great North Alabama scenery, including Little River Canyon, DeSoto Falls, Horse Pens 40, the Bankhead canyons, etc., etc., etc. – Jim Lacefield