The foundation for this unique landscape formed 450 million years ago when the area lay in a coastal zone and alternating layers of shale and limestone were deposited up to 400 feet thick. This was followed by a period when the area was covered by a broad, shallow sea filled with coral and myriad sea creatures. This period saw the development of a thick, fossil-filled limestone cap over the area. The final geologic act to shape our Tri-State region into what it is today saw successive advances and retreats of massive glaciers over much of North America beginning 1.8 million years ago. These glaciers pushed as far south as the current location of Ronald Reagan (Cross County) Highway in Hamilton County, scraping the landscape and blocking ancient rivers that flowed northward in the region. The damming of these rivers created massive reservoirs in the valleys of Eastern Kentucky and West Virginia. These reservoirs eventually broke through and carved the Ohio River Valley. When the glaciers receded, they left massive pockets of sub-surface gravel forming the Great Miami Buried Valley Aquifer System, an important source of drinking water for many communities north of the Ohio River. The melting glaciers also carved the innumerable stream valleys and deposited fine clay soils for which the region is known. The glaciers also provided rich soils present between the Ohio River Valley and the glacial Great Lakes.
North of the Ohio River, in areas underlain by deposits of Wisconsinan and Illinoian glacial drift, valleys are present near the major streams. Areas of older or no drift sheets, the landscape is more deeply dissected by streams. The steepest slopes are located below Cincinnati along the Ohio River, where the river impinges and undercuts a valley wall. This cut bank consists of soft Kope shales capped by more resistant limestones, which form bluffs with even greater slopes. Some of the valley sides of the Licking River have comparable slopes for the same reasons.
In contrast to the deeply dissected topography described above are the poorly drained flat uplands of Clermont and southeastern Warren counties. These areas are developed on the Illinoian till plain, which has had about 300,000 years for weathering to smooth and flatten its original glacial topography. The much younger Wisconsinan drift sheet, no more than 19,000 years old, commonly has a gently rolling topography and much better drainage. Both the younger and older drift sheets have modified the Tri-State’s terrain by filling its deep valleys with till and outwash.
Upland elevations across the region are uniform, ranging from about 825 to 900 feet above sea level. The highest elevations occur in northern and southeastern Warren County on scattered outliers of Silurian carbonates, which stand 35 to 55 feet above Ordovician sedimentary rocks. The local relief – the difference between the highest and lowest elevations in a county – is also restricted, averaging about 530 feet. In the Tri-State area and across the Cincinnati Arch, these upland elevations define a regional erosion surface described as the Lexington Peneplain.
Geology and hydrology have molded the landscape and driven the growth of our geographically gifted region. The OKI Regional Conservation Council is dedicated to preserving the benefits of our dynamic geological history.
A website and a printed publication served as the key sources of information for the geological history above. They are:
Ohio History Connection, Ohio History Central, Bedrock Geology of Ohio http://www.ohiohistorycentral.org/w/Bedrock_Geology_of_Ohio
Exploring the Geology of the Cincinnati/Northern Kentucky Region, Today’s Landscape and Its Heritage (pg. 12) by Paul Edwin Potter for the Kentucky Geological Survey, Series XI, 1996, Special Publication 22