The Geology of Abraham Lincoln Birthplace National Historical Park

This summary of the geology of Abraham Lincoln Birthplace National Historical Park in Kentucky draws on the National Park Service's Geodiversity Atlas, part of the Geologic Resources Inventory.

The park's resources

The park has two units:

• Birthplace Unit: the historic memorial and cultural landscape, Sinking Spring, old-growth forest and the site of the historic Boundary Oak.
• Boyhood Home Unit: a 3-hectare (7-acre) field, a rural 19th-century landscape, Knob Creek and its tributaries, rare limestone glades — open patches on south- or west-facing limestone slopes with shallow, rocky soils and plants adapted to harsh, dry conditions — unusually diverse and abundant plants, a boundary oak, fossil-bearing Mississippian limestone, and possibly a stretch of the original Louisville-Nashville Turnpike (Cumberland Road).

Knob Creek. Image from the National Park Service's page.

A karst plain on an ancient basin

The park sits on a Mississippian-age limestone karst plain, the Pennyroyal Plateau of central Kentucky, part of the ancient Illinois basin on the western edge of the Appalachian Mountains.

The Illinois basin is a spoon-shaped, lopsided depression trending northwest–southeast through Kentucky, Indiana and Illinois. At its deepest it holds more than 4,300 m (14,000 ft) of mostly shallow-marine Paleozoic sediment. It is ringed by uplifts:

Layers thin toward the basin's edges; the park lies on its eastern flank. Through most of the Paleozoic, especially in western Kentucky, the Rough Creek graben was the center of deposition. The basin has been repeatedly uplifted, warped, folded and faulted, and large structures such as the Rough Creek fault system helped shape where oil and gas collected. In the Paleozoic it was open to the south, as fine, deep-water sediments show, and it rose above sea level at least 50 times, with its margins exposed about 100 times.

Fossils

Though not formally documented, fossils are known from the park's bedrock, including crinoids and other Mississippian fossils in the Borden Formation; some crinoids may be partly intact and still show color banding. Brachiopods, bryozoans and corals are fairly common in the Borden Formation and may occur in the park, though the formation, widespread in the region, generally holds no rare fossils. Fossils on park land are protected by the Paleontological Resources Preservation Act of 2009.

The spring that chose a homestead

Karst shapes the park's groundwater and surface water, and may explain why the Lincolns settled here. Sinking Spring, a year-round karst spring flowing from a small cave, was the main reason Thomas Lincoln chose the site in 1808; reliable water is scarce on the Pennyroyal Plateau. The spring is now a key interpretive feature and visitor attraction.

Karst aquifers are especially vulnerable to pollution from diffuse sources such as farm runoff, because anything entering at the surface moves quickly through the system with little natural filtering. Agriculture, septic systems and roads are the main threats to groundwater around the park. Caves on park land are protected by the Federal Cave Resources Protection Act of 1988.

Regional setting and further data

The park lies in the Interior Low Plateaus physiographic province and shares its geologic history and several formations with the wider region. The Geologic Resources Inventory has produced a geologic map and report for the park; both are available on the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Abraham Lincoln Birthplace National Historical Park, Kentucky."
• Works it cites include Lawliss and Hitchcock (2004); National Park Service (2006); Maverick Energy, Inc. (2002); Swann (2007); Seadler and others (2002); Ryan and Meiman (1996); and communications from the Kentucky Geological Survey and National Park Service staff.
• A typing error in the source ("Indian" for Indiana) is corrected here.