Along the banks of the Susquehanna River near Falmouth lies one of the most remarkable geological sites in Lancaster County—Conewago Falls. Here, at a natural constriction in the river valley just south of Three Mile Island, a rare display of potholes, sculpted rock formations, and displaced boulders tells the story of both slow erosion and sudden catastrophe.
The bedrock at Conewago Falls is a dark, Jurassic-age igneous rock known as diabase. Formed over 200 million years ago during the breakup of the supercontinent Pangaea, this hard, erosion-resistant material is ideal for preserving the subtle and not-so-subtle effects of waterborne force. When the river’s gage height at Harrisburg drops below 3.5 feet, large portions of the exposed riverbed reveal a sculpted surface filled with potholes, ripple marks, and smooth troughs.
These potholes, often 1 to 2 feet across—though some are much larger—were formed by fast-moving, sediment-laden water swirling over weak points in the rock. As sand and quartz particles spun in vortices, they gradually drilled into the diabase like nature’s own sandblasting machine. The effect is both visually striking and scientifically significant, offering insight into the tremendous erosional forces at work in this stretch of the river.
But the story of Conewago Falls doesn’t end with steady erosion. It also includes a dramatic chapter from the Ice Age.
Geologists believe these features were shaped, at least in part, by catastrophic glacial outburst floods that occurred tens or even hundreds of thousands of years ago. During one or more pre-Illinoian glaciations, a massive ice dam formed along Bald Eagle Mountain in central Pennsylvania. This blockage created glacial Lake Lesley, a body of water that may have held up to 26 cubic miles of meltwater.
When the ice dam eventually failed, it sent a torrent of water—possibly more than 600 feet deep—racing down the Susquehanna River. As the floodwaters surged through constrictions like Conewago Falls, their velocity increased dramatically, scouring the diabase bedrock and dislodging massive boulders, some weighing over 100 tons. Many of these displaced blocks remain near the site today, visible reminders of a long-lost moment of geological violence.
The unique composition of the diabase at Conewago enabled these features to survive, whereas similar evidence elsewhere along the river was eroded or buried by later events. Today, this stretch of riverbank offers one of the clearest records of ancient flood processes in the mid-Atlantic region.
Whether you’re a geologist, naturalist, or curious adventurer, Conewago Falls provides a rare opportunity to step into the aftermath of a cataclysm, frozen in stone.
Locating the Conewago Falls
The Conewago Falls are located directly upriver from the Pennsylvania Fish and Boat Commission parking and boat launch in Falmouth. Google Maps also lists the location as the Falmouth Trailhead. From the boat ramp, when facing the Susquehanna River, turn right and walk along the shore upstream. The unique potholed diabase boulders continue to the county line.
Note Well
Conewago Falls is best visited during periods of low water, ideally when the Harrisburg river gage reads 3.5 feet or lower. Start at the Falmouth Fish Commission Access just off PA-441. From the parking lot, walk north along the exposed riverbed to view the potholes and rock features. For safety and access updates, check the real-time river height via the USGS Stream Gage 01570500. Always exercise caution when walking near or on wet rock surfaces.
Learn More
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Packed with history, local stories, and GPS locations, this book is your ticket to exploring the mysterious corners of Lancaster like never before. Whether you’re a lifelong local, a history buff, or just looking for a unique adventure, this field guide will spark your curiosity and send you exploring. Start your adventure here.
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