At the end of the last ice age 18,000 to 15,000 years ago massive hydrologic floods ran down the Columbia River Gorge.  Originating from a huge glacial lake near Missoula, Montanta around forty floods were large enough to flood the entire Willamette Valley to a depth of 400 ft.  These floods did not just bring huge volumes of water, they brought huge amounts of rocks and sediments from the continental batholiths.  A large percentage of these rocks and sediments are composed of granite.  Granites from the Idaho batholith are high in uranium bearing minerals.  This high uranium content brings with it an unexpected hazard, radon.