Granitic rock exposed at the Dells is Proterozoic in age (1,400 Ma). The granitic melt was intruded into the ancient crust where it cooled and crystallized. It has since been exhumed and raised to the surface. The rounded appearance of individual boulder is the result of spheroidal weathering - a process whereby minerals, feldspars and micas, are chemically attacked along joints, effectively rounding a square peg. Radon concentrations are anomalously high in the Dells due to a high concentration of uranium.