As you float down Cypress Creek or relax in Wildwood Park, notice the bluffs of gray rocks along the water. The rocks along Cypress Creek formed during the Mississippian Period (359-323 million years ago) of geologic time, just after the continent Laurentia was created from the joining of what is now North America and parts of northern Europe. At this time, sea level was extremely high and much of Laurentia was covered by a shallow tropical ocean. At the start of the Mississippian Period, Alabama was just south of the equator and offshore of the Laurentian landmass, under a warm shallow sea. To the south was a narrow ocean basin called the Rheic Ocean. The Rheic Ocean separated Laurentia from the southern continental landmass of Gondwana, which was moving northward to eventually collide with Laurentia, forming the supercontinent, Pangea. The rocks we see in North Alabama document this major geologic event.
A tropical sea covered Alabama for about 40 million years (Ma) forming the marine rocks you see in this area today. The oldest of these is the Fort Payne Chert (355-347 Ma; 100-150 feet thick) exposed along Cypress Creek and in the bottom of the bluffs on the south side of the Tennessee River in Sheffield. Chert is a microcrystalline quartz that precipitates out of ocean water to become rock. Chert is very resistant to erosion and eventually accumulated on the bottom of the Tennessee River, resulting in river shoals and lending the name to the city of Muscle Shoals.
Above the Fort Payne Chert is the Tuscumbia Limestone (347-337 Ma; 200 feet thick). This formation is exposed again in the Tennessee River cliffs as well as along of the top Cypress Creek cliffs. The Tuscumbia Limestone is also a marine rock deposited in warm shallow seas, abundant with life. You will find many fossils in this unit including crinoids and corals. Unlike chert, limestone erodes very easily. Millions of years later, the Tuscumbia Limestone provided fertile soils and fresh water springs that attracted settlement to the area.
Most of the fossils found in North Alabama are Mississippian Period ocean life. Alabama was covered by a shallow, sunlit ocean abundant with life. Common fossils include echinoderms like crinoids and blastoids that were attached to the ocean floor. Both animals had tentacles at the top of a stemlike structure used to catch food. Commonly, you will find the remain of the crinoid’s “stem” which look like a disc with a hole in the middle. The body of blastoids are often preserved and resemble a flower bud shape with four to five feathery structures at the top. Their stems were smaller in height and diameter and lack the hole seen in crinoid stems.
If you find a fossil or something else that inspires your inner geologist, share with us by tagging us on social media (@muscleshoalsnha) and using #ourcypresscreek!