Several small depressions along the Hells Hollow Trail are sinkholes. Like the caves and flume, they are formed by the dissolution of limestone by acidic water. These features are very common in areas underlain by limestone.
There are also other depressions near the trail which are not sinkholes. There was much industrial activity in this valley in the 1800s (iron furnace, lime kiln, limestone quarry, coal mine). Some of the depressions are man-made.