Catastrophic floods built Grand Canyon UPI Environment News From the Science & Technology Desk Published 7/20/2002 6:00 AM GRAND CANYON NATIONAL PARK, Ariz., July 20 (UPI) -- Dams of volcanic rock laid across the Grand Canyon have burst repeatedly and catastrophically over the past million years -- most recently about 165,000 years ago -- carrying enormous onrushing floods and carving out much of the great landmark in the blink of a geologic eye, new research by U.S. Geological Survey and University of Utah geologists suggests. The findings tend to support other new data indicating the canyon's Inner Gorge may...