Geology Hike: Sally Brown/Crutcher/Wallace Nature Preserve

By: Kenneth Brooks, Volunteer Nature Preserve Steward

Earlier this month, Marty Parris from the Kentucky Geological Survey led a hike and discussion of geology at the Conservancy’s nature preserve. Participants included members, guests, board members, staff and volunteers.

The hike began with a brief overview of The Nature Conservancy’s efforts to manage and monitor for this nature preserve. Marty then began an interesting and insightful review of geology in general, and then drilled down to the local area beginning with a hike which took the group from the parking lot to the Kentucky River.

Along the way, Marty explained and illustrated in an excellent handout the nature preserve’s geological features. We learned that it is located generally in the center of a large underlying feature called the Cincinnati arch, a dome-shaped structure on top of which billions of years of sediment has been deposited.

Running through the area is a significant fault that is not visible within the nature preserve but is quite obvious on US 27 near the river where the layers of limestone change from horizontal to almost vertical. While very apparent and interesting, we were all glad to hear the fault is also very old and there is no record of any movement. In fact, it is so old that there is no telling exactly what caused this fault.

Our group also learned that all of the rock layers in the immediate area of the nature preserve are limestone and are Ordovician in age, 448 to 460 million years old. They represent some of the oldest exposed rock in Kentucky and the eastern United States. There were three primary layers of this rock found at the nature preserve.

The first layer, observed at the parking lot, is the lower level of Lexington Limestone strata. It is visible in slabs of rock in the field north of the parking lot and on the hillside as you follow the trail into the woods. This strata is 180-210 feet thick, fine-grained limestone and shale, and often contains fossils.

Geology Hike 4Between the first and second layers is bentonite and ash deposited millions of years ago across the United States in a huge volcanic eruption. The bentonite and ash tend to prevent up and down movement of water and encourage horizontal movement, accounting for the four springs in the wooded area on the right side of the trail as you go through the fence near the nature preserve kiosk. (The largest of these springs is perennial and in proximity to an old cattle watering cistern.) The presence of the bentonite and ash also helps in dating the age of the rocks.

Marty informed us that the second layer is Tyrone Limestone and the Oregon Formation, 100-140 feet thick, fine-grained limestone, with few fossils. You walk through this layer on the Brown Trail leading from the kiosk to the picnic table where the trail goes down to the river.

The third layer is the Camp Nelson Limestone below the picnic table to the river bank. This is about 280 feet thick, mottled, and fossiliferous. It is largely what you see in the cliffs of the Palisades.

Near the picnic table on the Brown Trail and near the base of the cliffs in the river bottomland are many small and large sink holes. These are where fractures in the limestone are allowing erosion to form depressions and also dry and perennial creeks.

Marty Parris from the Kentucky Geological Survey led a hike and discussion of geology at one of the Conservancy’s Kentucky River Palisades nature preserves.
Marty Parris from the Kentucky Geological Survey led a hike and discussion of geology at one of the Conservancy’s Kentucky River Palisades nature preserves.

While interesting as geology features, the limestone Palisades (cliffs in this area 300-400 feet tall) also have impacted development. For example, the limestone is valuable for building material, especially as gravel, and contributed to our horse and bourbon industries. The cliffs themselves also create unusual microenvironments that help to account for our diversity of flora and fauna, including some that are very unique and special. They have limited human development resulting in the rather pristine stretch of wooded river bank now protected by this nature preserve as other efforts by The Nature Conservancy.

It was a beautiful fall day to learn about the geology of the nature preserve. Thanks to Marty for his excellent commentary.