Geology
The precambrian Dells' granite has been dated at 1.4 billion years old. Its pluton was intruded at a depth of around one or two miles (1.6 to 3.2 km). These cover rocks have since been eroded away. Weathering along joints produced the rounded boulders and other unusual rock formations that characterize the Granite Dells. This process is called spheroidal weathering, and is common in granitic terrains.
The Dells' granite has an unusually high uranium content, and thus homes built over the granite should be checked for radon leaking from the granite.
Read more about this topic: Granite Dells