A Sea of Life – The Mississippian Period
During the Mississippian Period (359–318 million years ago), a shallow inland sea covered much of what is now the Ohio valley of North America. These warm, tropical waters teemed with life, hosting an array of invertebrate organisms that extracted calcium carbonate from the seawater to form their shells and skeletons. Bryozoans, brachiopods, corals, and echinoderms (especially crinoids) were abundant in this thriving marine ecosystem.
From Ocean Floor to Limestone
As these animals died, their shells and skeletal remains settled to the ocean floor, forming layers of white sand. Over time, these deposits mixed with calcium carbonate crystals that naturally precipitated from the seawater. Ocean currents and wave action sorted tiny rounded shells of foraminifers, small fossils, and fragments of larger shells into uniform sizes. This natural sorting gave the thick deposits their distinctive homogenous texture. Over millions of years, the accumulated marine remains were compacted and cemented.
At the outset of the Quaternary Period about 2.58 million years ago, the continental land masses resembled closely what we see today and the onset of glaciation in the Northern Hemisphere began.
Several glacial events occurred from this period. From around 13,600 years ago as the glaciers retreated, melts from the Laurentide Ice Sheet resulted in epoch flooding events that carved through much of the area just north of Bloomington, exposing a great deal of the underlying geologic formation along stream beds and outcroppings across south-central Indiana and exposing the deposit of the Mississippian age, we now know as Salem Limestone.
Next, we’ll study how this deposit was discovered and how this stone became the foundation of an industry that shaped both Indiana’s history and America’s architecture…..Stay tuned!