How rivers carve round potholes into solid rock

A smooth round hole in solid rock can look drilled. In a riverbed, flowing water and the sediment it carries can produce that effect without a metal tool. The hole records abrasion: rock wearing against rock.

The useful question is where the hole formed. A stream pothole and a rainwater-filled hollow may share a rounded outline while recording different processes.

A moving river supplies the abrasive material

The Minnesota DNR account of Interstate State Park describes more than 200 potholes in basalt, formed where water and sand swirled in the much larger glacial St. Croix River roughly 10,000 years ago. Nearby examples include the Bake Oven and Devil’s Parlor; the latter developed from smaller potholes that the river enlarged into an open space.

Imagine the abrasive material as part of the flow, rather than water acting like a perfectly clean drill. The round surface is a result of repeated wear. Its appearance alone cannot tell you how long the process took or how quickly a particular hole deepened.

Why some potholes stand above today’s water

At Great Falls, the National Park Service identifies the River Trail as part of an ancient Potomac riverbed, now about 40–75 feet above the present river. Rounded boulders, smoothed surfaces, grooves and potholes supply clues to that earlier position. The river subsequently cut into fractured rock along Mather Gorge.

A dry pothole therefore need not have formed without a river. The landscape and channel can change after abrasion creates the hollow. Separating formation from later exposure avoids the mistake of treating today’s water level as the water level throughout the site’s history.

A rainwater basin tells another story

At City of Rocks, the Three-Pool Boulders explanation calls the granite hollows panholes. Water stays in small depressions or cracks; acidic water dissolves minerals over time. The same account distinguishes a stream pothole, made by stones grinding in a rocky channel, and says no such potholes are known within that reserve.

This distinction concerns process, not simply vocabulary. Calling every circular hollow a river pothole can insert a vanished river into the story without evidence for one.

Read the setting before naming the hole

• Surface: look for abrasion clues such as smoothed rock, grooves or rounded stones. A shape alone is an incomplete diagnosis.
• Position: ask whether the surrounding surface is interpreted as a current or former riverbed.
• Context: use local geological interpretation to distinguish stream abrasion from weathering in standing water.

These are questions to guide observation, not a field test that reliably dates a hole from a photograph. The three sources describe different places; their ages, rock types and water histories cannot be swapped between them.

Rock can acquire an orderly shape through other processes too. Basalt columns begin with cooling fractures; a river pothole is an erosional feature cut into an existing surface.

References

• Minnesota Department of Natural Resources. (n.d.). Interstate State Park: Snapshot virtual tour. https://www.dnr.state.mn.us/state_parks/virtual_tour/interstate/dialup.html
• National Park Service. (2015, April 10). Geology. https://www.nps.gov/grfa/learn/nature/geology.htm
• National Park Service. (2021, April 10). Three-Pool Boulders. https://www.nps.gov/places/three-pool-boulders.htm

Reading notes: Minnesota DNR and NPS provide separate location-specific interpretations. No field measurements or universal erosion rates are inferred. The DNR virtual tour has no displayed publication date; the other dates are page updates.