Geologic Province
Appalachian Plateaus Province – Allegheny Plateau Section
Introduction
This EarthCache highlights a classic example of differential erosion in the Laurel Highlands of Pennsylvania. The waterfall and rock overhang at this site exist because layers of sedimentary rock erode at different rates. By observing the rock layers, waterfall shape, and surrounding landscape, visitors can see how ancient environments and modern erosion combine to shape today’s terrain.
Geologic Background
The bedrock at this location formed during the Pennsylvanian Period, approximately 300 million years ago, when this region lay near the equator. At that time, rivers, deltas, and coastal swamps deposited layers of sand, mud, and organic material.
These sediments eventually hardened into:
- Sandstone (from sand deposited by rivers)
- Shale (from mud settling in quieter water)
- Thin coal layers (from buried swamp vegetation)
Today, these rocks lie nearly horizontal, a hallmark of the Allegheny Plateau, unlike the folded rocks found farther east in the Appalachian Mountains.
Why There Is a Waterfall Here
The waterfall exists due to differential erosion:
- Sandstone is more resistant to erosion and forms the cliff and overhanging ledge.
- Shale, which lies beneath the sandstone, erodes more easily when exposed to flowing water and freeze–thaw cycles.
As the shale wears away, the sandstone loses support and breaks off in blocks, causing the waterfall to slowly migrate upstream over time. This process also creates the recessed area behind the falling water.
What to Observe at the Site
At the EarthCache location, visitors can clearly see:
- Horizontal rock layers, representing different ancient depositional environments
- A sandstone caprock forming the waterfall lip
- Softer shale layers below the sandstone
- Evidence of erosion, including rock fragments at the base of the falls
- A human-built walkway that allows close, safe observation of the rock face
EarthCache Logging Requirements
To log this EarthCache, please message your answers to via geocaching email or the caching messenger.
Do not post them in your log.
- Rock Identification
From visual observation, describe two differences between the upper rock layer and the lower rock layer (color, grain size, hardness, or erosion style).
- Erosion Evidence
What evidence do you see that the waterfall is eroding the rock over time?
- Waterfall Shape
Explain why the waterfall has an overhang instead of a smooth sloping rock face.
- Photo Requirement
Post one photo showing you (or a personal item) at the EarthCache location. Please do not include faces if you prefer privacy.
Safety and Accessibility
- Stay on designated walkways and platforms.
- Rock surfaces may be slippery due to spray.
- Do not climb on rock faces or attempt to access restricted areas.
References:
- Pennsylvania Geological Survey – Geology of Pennsylvania
Overview of Pennsylvania’s bedrock geology, geologic history, and major rock types, including Pennsylvanian-age sedimentary rocks.
https://www.pa.gov/agencies/dcnr/conservation/geology/geology-of-pa.html
- Pennsylvania Geological Survey – Physiographic Provinces of Pennsylvania
Describes the Appalachian Plateaus Province (including the Allegheny Plateau and Laurel Highlands) and the relationship between landforms and underlying geology.
https://www.pa.gov/agencies/dcnr/conservation/geology/geology-of-pa/landforms.html
- Pennsylvania DCNR – Trail of Geology Series
Educational publications explaining Pennsylvania landforms, sedimentary rock layers, and erosion processes observable at public sites.
https://www.pa.gov/agencies/dcnr/recreation/where-to-go/scenic-features/geologic-features/trail-of-geology.html
- Geology.com – Appalachian Plateau
General overview of the Appalachian Plateau, including horizontal sedimentary strata, sandstone and shale composition, and erosion patterns.
https://geology.com/physiographic-provinces/appalachian-plateau.shtml
- Geology.com – Sedimentary Rocks
Explains sandstone and shale formation, properties, and resistance to erosion, which are key to understanding waterfall formation at this site.
https://geology.com/rocks/sedimentary-rocks.shtml