One of the most curious features of Floyds Fork is the shape of its floodplain when viewed over the length of the stream. It starts narrow in the north, and then becomes wide in The Parklands section, before going narrow in a steep valley in the south. One can see this by looking at a topographic map, a geological map, or even Google Maps using the terrain feature. Better yet, would be a hot-air-balloon ride over Floyds Fork.
Whichever way you choose to explore you’ll notice that the floodplain is widest in The Parklands section of the Floyds Fork. Here, the floodplain averages 2600 feet (~0.5 miles) wide. To the north (of Shelbyville Road), though subtle in topographic relief, the floodplain is much narrower. And to the south (of Bardstown Road) the floodplain is much narrower than in The Parklands—just 500 feet—but the valley walls are much steeper and higher. Why is this?
Geology controls the topography. Remember that rocks in this region, and all of Kentucky, are sedimentary rocks laid down hundreds of millions of years ago as sand, clay, marine life, or plants that today form sandstone, shale, limestone, or coal respectively. Different rock layers erode at different rates depending on what they are made of and how tightly bonded they are. Soft layers erode rapidly while solid layers resist erosion.
Also important in this story is a feature called the Cincinnati Arch. Imagine a giant fist several miles beneath Lexington pushing up to form a bend or gentle dome in the rock layers. Then scrape the top few layers off and you have the old limestones that form Kentucky’s Bluegrass Region. The Arch shows up around Louisville as a slight dipping of the rock layers to the west. That is, all the rock layers of Floyds Fork tip downward toward downtown Louisville. Because the top has been eroded off the arch, younger rocks occur to the west, and older rocks to the east.
As Floyds Fork begins its journey in Oldham County it flows south cutting through various layers of rock. A bit north of Shelbyville Road Floyds Fork cuts into a soft limestone layer called the Rowland Member. This soft limestone and shale erodes very easily—it forms dozens of waterfalls in the southern parklands and interesting formations on Turkey Run and Old Man’s Run. Once Floyds Fork cut into this layer, and the Grant Lake Member below it, it likely eroded the valley walls quickly, widening the floodplain as it carved meander after meander.
Looking at geological maps (http://kymartian.ky.gov/gqmaps/) one can see the Rowland layer continuing to the area around Bardstown Road. The Grant Lake ends just north of it. You can also see the shapes of the valley walls show ancient meanders of earlier courses of the Fork. Notice the semicircles that incise into the valley walls; these are ancient meanders.
At Bardstown Road things change. In this map of the southern parks Bardstown Road bisects the map and the Fork’s general flow heads west through many meanders. Because the rock layers dip west, Floyds Fork begins to climb out of the lowest geological layers, the Rowland, and move up through younger and younger rock. The rock layers above the orange Rowland are more resistant to erosion so the valley is tighter and deeper. From the orange Rowland Member, layers above it include the red Bardstown Member and the pink Saluda, closing out the Ordovician-aged rocks. Silurian rocks include the light purple colored Osgood and Brassfield formations, the purple Laurel Dolomite, dark purple Waldron shale, and the blue Louisville Limestone, the last Silurian aged rock in Louisville.
Thus the formation of the wide floodplain was likely caused by the generally weak Rowland and Grant Lake layers which allowed Floyds Fork to rapidly erode the valley walls in The Parklands section where those layers are exposed. Then, because of the slight dip of the rocks to the west caused by the Cincinnati Arch, Floyds Fork becomes locked into the younger, more resistant Silurian aged rocks after turning toward the west around Bardstown Road. This geologic story also determined the agricultural history of the landscape and, by extension, The Parklands project itself.
About the Author
Michael Gaige became involved with 21st Century Parks in 2007 on a recommendation from his graduate adviser, Tom Wessels, author of Reading the Forested Landscape. His first project was to groundtruth the nearly 4,000 acres around Floyds Fork to discover and document interesting places park users will experience and learn about. He then compiled a Natural Areas Plan to ensure that the parks’ forests and meadows are well-tended, and park infrastructure is designed in accordance with the landscape’s history and ecological detail. Michael now works as a freelance ecologist and educator and lives in upstate New York. He returns to Louisville periodically to share with others his favorite places in The Parklands, and to visit his cherished old trees.