The geologic foundation of Louisville contains beautifully stacked layers of sedimentary rocks, much like the Grand Canyon, just not as deep. Limestones, shales, and sandstones deposited 450 million to 300 million years ago during an era called the Paleozoic pressed into the rocks that form Floyds Fork to Iroquois Hill. Many of these rock layers contain abundant fossils—the Falls of the Ohio is one example, and one of the richest fossil sites in the world.
The oldest rock layer exposed in Jefferson County occurs at the stream bed of Floyds Fork, coincidentally, between Shelbyville Road and Bardstown Road, the project area of The Parklands. Known as Grant Lake Limestone (formerly known as the Arnheim Formation) the sediments were deposited during late Ordovician times, about 450 million years ago when what is now Kentucky occurred south of the equator as a shallow sea.
From the banks of Floyds Fork imagining a shallow sea is not difficult. The abundant Platystrophia ponderosa fossils readily showcase the ancient marine environment. Playtstrophia ponderosa is a brachiopod—a marine animal concealed in hard shells, or valves. These charismatic fossilized creatures can be as large as a golf ball, but are usually about half that size. Though brachiopods still exist on Earth, most groups died out during an extinction event 250 million years ago. Playstrophia ponderosa likely died out in separate great extinction event 440 million years ago, at the end of the Ordovician period.
Today, in addition to forming the bedrock under Floyds Fork in Jefferson County, Grant Lake limestone also appears in southern Ohio, Indiana, Tennessee, and on the east side of Kentucky’s bluegrass region.Platystrophia ponderosa can be found at many of these sites as well. Geologically, the Bluegrass Region is an arched dome with the top sheared off called the Cincinnati Arch. Thus many rock types circle the region as concentric rings roughly centered on Lexington. The Grant Lake is also commonly exposed in uplands east of Jefferson County.
Grant Lake limestone is relatively hard (to break by hand), but because it is interbedded with shale it is easily broken up by stream forces and weathering. As the stream bed layer, the Grant Lake forms the riffles, rapids, and pools along Floyds Fork due to its many beds. Because these beds are thin (typically 1 to 4 inches) the riffles in Floyds Fork are small; there are no large drops or waterfalls on Floyds Fork itself in The Parklands. Because it is the stream-bed layer and readily breaks up into manageable chunks, Grant Lake limestone was also the dominant stonewall building material in the Floyds Fork area; stonewalls commonly line the banks.
Join us for a canoe trip this spring, or paddle your own boat down Floyds Fork beginning at Miles Park. Any gravel bar will undoubtedly reveal Platystrophia fossils, and the rocky outcrops along the bank will be Grant Lake limestone. By the time you paddle past Broad Run Road you begin to leave Grant Lake rocks. Platystrophia fossils, however, continue in the gravel bars all the way to the Ohio River, and beyond.