Following the ice age period — eons ago, our local area was covered by a relatively shallow tropical ocean. Biological limestone is sedimentary organic rock formed by the accumulation of calcium carbonate shells, coral, algae and skeletal debris. Their biological origin is often revealed in the rock by the presence of marine fossils.
Many of these fossils are known as cephalopods – looking like giant octopuses or squid in hard “ice cream cone” shells. As these and other shellfish died, they left great thicknesses of sediment, forming layers of stone, in which many beautifully preserved seashells can still be seen from the time of ancient Illinois oceans.
In 2009, R.J. Spillane, the founder of what is now the Stagecoach Trails Limestone quarry, discovered a 5-foot-long fossil of a cephalopod. When it lived, the mollusk had a soft base body and a hard shell. The fossil protrudes outward and is easily visible on the surface of the 2,100 lb. rock retrieved from our quarry in Apple River, Illinois. The specimen is now on display at a nature center in the Chicago area.