Much of Minnesota sits atop old volcanic rock. Is some places where water runs over the basalt rock, a small depression may cause the water to swirl over the depression. Sand in these swirling areas will wear away at the rock causing the depression to grow deeper and deeper. This forms a “pothole” which can bee seen when the river dries up or shifts its path. This is inside one of the larger potholes that can be found at Taylors Falls.