In a very real sense, this spot is the heart of the Quartzville mining district. The most intense and valuable mineralization is still several miles away, around the historic town site, but the intrusive rock here, diorite, was the source of heat that created the hydrothermal system that mineralized the area in the first place. The rock is dated at about 18 million years, so that was when the alteration took place. I suspect a fault created a weak spot that allowed erosion along the fairly linear drop of the falls, but as we'll see later, basalt dikes cut through this area, and are preferentially eroded by high winter and spring flows. It's possible one such dike allowed the erosion of that step.
There are two holes here that are nice for swimming, or "paddling," as we called the mixture of swimming, wading, and crawling around in shallow water when I was younger. The cascade above falls into the upper hole, and you can push yourself back under that tail, still able to see what's going on out on the rocks pretty well. I'd love to just sit there and wait for someone to show up... then spring out with a yell. Probably get myself shot.