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Geology meets art at pothole playground on Susquehanna in Lancaster County [photos]

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In extreme northwestern Lancaster County, when the Susquehanna River recedes, as it has now, careful exploring on foot reveals an otherworldly canvas where geology meets art.

Here, at the Conewago Falls near Falmouth, Conoy Township, just below Three Mile Island, molten rock, ruptured glacial lakes and thousands of years of erosion have left behind a huge rock garden of potholes, sculpted boulders and undulating scour marks.

The price of admission is free, and it’s all easily accessible, though caution is strongly advised while picking your way through the jumble in search of works of art. The surface is uneven, and the rocks can be slippery.

This field of sculpted river rocks, along with others below the Holtwood Dam, are considered the best displays of such phenomena in the entire country.

Still, the wonderland is relatively unknown by many in the area. You’re more likely to see anglers drifting in boats and kayakers leisurely paddling in the center channel than observe people slowly picking through the playground of rocks. I was the only person not in the water on a recent weekday morning.

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However, in October 1947, when the full extent of the rocks was unusually revealed during low flows on the Susquehanna, an estimated 10,000 people flocked to see the spectacle that stretched across the entire river, according to a Lancaster newspaper.

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The rocks have been called “sensuous” and “tortured” and “exotic” by writers through time. Former state geologist Donald Hoskins described them as a “river on a rampage.”

They also have been referred to locally as “hunger rocks” because they are most visible during times of drought, times not favorable to farmers.

Just what are we looking at?

The rocks are all diabase, a super hard and dark igneous rock. They are a molten rock left here about 200 million years ago.

The rocks spread out from both shorelines, pinching the width of the Susquehanna at this point by about two-thirds.

During the Ice Age, the more intense flows at the Conewago Falls caused underwater tornadoes, allowing bits of glacial sand and debris to slowly swirl in circles, sand-blasting their way through joints in rocks. Over thousands of years this created potholes, probably the number one attraction for modern-day explorers at the Conewago Falls.

They can be perfect holes, sometimes drilled all the way through rocks, to oblong and irregular troughs in boulders holding water and grasses.

If you search long enough, you can find some the size of a tub and filled with warm water and a pebbly bottom of shells, snails and crayfish claws.


Algae finds a home in a river pothole.

Nearer the main channel in the center of the river, look for ripples on the bedrock, waves formed by sand eroding the rock.

Other than the potholes, I was most transfixed by the rounded, sculpted tops of rocks that flow together like soft mountain peaks. These are the sand-scarred ripples eroded and rounded over time, up to about 20,000 years ago.


Sculpted rocks from the Ice Age at the Conewago Falls. In the background is the York Haven Dam.

The biggest wow factor on the landscape, though, are those occasional truck-size boulders you see amid the rocky landscape.

They were slid and rolled around during one monumental event during the Ice Age, about 750,000 years ago, when a huge ice dam that had blocked the West Branch of the Susquehanna near what is now Williamsport gave way.

The resulting wall of water was still perhaps 100 feet high when it raged through the gorge of the Conewago Falls with renewed vengeance.


A huge rock moved by the collapse of a glacial lake on the Susquehanna about 750,000 years ago.

How powerful was it? Consider that flows during Tropical Storm Agnes were one-fourth that of this inundation.

You’ll see that some of the rocks have potholes in their sides, formed before the rocks were thrust topsy-turvy.

Everything you now see remains pretty much as it was after that powerful event. No floods since then have come close to rearranging the rocks.


Potholes and sculpted rocks on display during low water on the Susquehanna River.

Debris carried down the Susquehanna now is mostly mud, not the sand needed to further scour the potholes. And, thanks to dams, floods don’t last long enough to touch off the underwater tornadoes.

Unfortunately, the rocks have not escaped the scourge of spray painters. One of the most prominent boulders in the midst of the rock field is heavily defaced on top and one side.

Notice the grasses and plants that soften the landscape of rock. They are the few species that can withstand the constant immersion at the falls. Rose mallow, for example, has a taproot to keep the plant from being uprooted during high flows.

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Even though a large Susquehannock village was located near the Conewago Falls, don’t expect to find Native American petroglyphs carved into rocks, as found on rocks of schist in the river from Turkey Hill to Conowingo, Maryland. The diabase rock is simply too hard, notes Paul Nevin, a Wrightsville man and authority on Susquehanna River rock art.

However, Dave Gerber, a 64-year-old Conoy Township man who has poked around the Conewago Falls all his life, once found a rock with “George Doyle, Sept. 29, 1917” carved into it.