Geology

BN-Karst

Geology of the Bluegrass and Nashville Basin

The Bluegrass and Nashville Basin are both perched on domes of limestone, called the Lexington Dome and Nashville Dome.  The limestone formed in shallow seas during the Ordovician Period, 485-443 million years ago.  The Bluegrass and Nashville Basin are surrounded by younger rock through which the domes rose.  The Inner Bluegrass, Inner Nashville Basin, and Outer Bluegrass are nearly pure Ordovician limestone, while the Hills of the Bluegrass and Outer Nashville Basin are dominated by limestone with outcrops of shale.

Elevation profile
Elevation profile of the Bluegrass (upper) and Nashville Basin (Lower)

Although both the Bluegrass and Nashville Basin are similar domes of limestone, the Bluegrass dome has remained above the surrounding landscape, while the Nashville Basin is below the level of the surrounding landscape.

Limestone slowly dissolves as rain percolates through the rock. This creates a condition known as karst, in reference to the Karst Plateau of Slovenia and Northern Italy. Rain slowly dissolves the rock, creating cracks, fissures, sinkholes and caves.  The many famous caves of Kentucky, including Mammoth Cave and Carter Caves, are in these limestone formations, although outside of the Bluegrass. There are numerous small caves in the Inner Bluegrass and the Inner Nashville Basin.

The sinkholes of the Inner Bluegrass are one of the dominant features of the pasture landscape. Small sinkholes are also found throughout the Inner Nashville Basin.  Limestone is quarried in both the Bluegrass and Nashville Basin.

In spite of our abundant rainfall, our streams often dry up quickly after rain, and pastures become very dry. Karst allows rain or melting snow to percolate quickly through the soil into the underlying rock. Instead of forest and crops, the Inner Bluegrass and Inner Nashville Basin are dominated by grasslands. Our trees are very drought tolerant and often rooted very deeply.

Map of lithology of Bluegrass and Nashville Basin
Major surface rock formations of the Bluegrass and Nashville Basin. Dark blue = limestone; light blue = mixed limestone/shale

Map of karst (blue) in the Bluegrass and Nashville Basin
Map of karst (blue) in the Bluegrass and Nashville Basin