Stones trapped swirling in the roiling Deerfield River have been grinding into the rock bed at this location ever since the ice age. The result: 50 near-perfect circles in the riverbed, including the largest known glacial pothole (39ft diameter) in the world.
A hydroelectric dam overlooking this site now controls the flow of the river over the potholes, so it's possible that on your visit the water will be completely obscuring the holes. Either way it's worth a look – if the flow is a trickle, you readily see the circles; if it's raging, you'll feel like you're at Niagara Falls. The potholes are at the end of Deerfield Ave.
