Toroweap, Looking Down-Canyon
At Toroweap, a remote overlook on the North Rim of the western Grand Canyon.
Over the past million years or so, numerous volcanoes have erupted near here (along the Toroweap Fault) repeatedly pouring massive amounts of lava into the canyon. The river was dammed by the lava here about 650,000 years ago, forming a giant reservoir:
"Geologists estimate that the Prospect Dam lake took more than twenty years to fill with water. An incredible waterfall must have formed as water spilled over the lava dam. Over the thousands of years that followed, erosion by the rushing water undercut the softer river sediment beneath the lava dam, and also plucked and carried away the jointed lava itself. With time, the waterfalls got higher and higher as they eroded upstream toward the dam. Ultimately, a new gorge was eroded through the lava dam, leaving only the remnants of the basalt clinging to the walls of the canyon."
-From "Hiking the Southwest's Geology: Four Corners Region" by Ralph Lee Hopkins