The Jeffers Group is mainly a sequence of volcanic rocks and sandstone and shale . In the vicinity of Jeffers Brook, coarse grained volcanic rocks are exposed at the falls and in the cliffs adjacent to the plunge pool. Further upstream sandstone and shale beds are exposed in the brook.
Volcanic rocks originate in several ways.
The rock fragments are composed of basalt , andesite , felsic volcanic rocks , granite and others. Almost all of the fragments are angular and are not touching each other. No layering is visible. The maximum thickness of these rocks could be as much as 50 to 100m. Jeffers Brook is a V-shaped, subsequent stream that formed after the last glaciation.