[paleotrope]

The uranium and other radioactive substances are in the granite the north eastern states. I think the operative theory is the successive glaciations pushed most of the upper levels of the soil south, exposing the granite. Over the last 20k years, the soil is rather thin so you will see homes with basements that are not far from the granite. Radon then comes up and collects in these basements that aren't well ventilated.