Carved into the volcanic basalt of Idaho’s Malad Gorge State Park are two curiously shaped canyons: Woody’s Cove and Stubby Canyon. Each has high vertical walls that curve at the canyon’s head to form a U-shaped amphitheater whose origin has, until recently, been a mystery.
Geologists have assumed this rare shape is formed through “groundwater sapping,” where springs at the base of the canyon slowly eat away at the bottom of the rock wall till the structure becomes destabilized and large chunks of rock fall from the top of the wall (a process known as undercutting). Two geologists from Caltech, however, present evidence for an alternate, more dramatic, explanation: megafloods.
The canyons were likely formed at the same time, as a single huge flood inundated the area about 50,000 years ago, write Michael Lamb and Benjamin Mackey in a recent PNAS Early Edition. This flood, they estimate, which was strong enough to wash away enormous boulders, moved at least 330,000 gallons per second and was a minimum of 30 feet deep.
While there are springs at the base of Woody’s Cove and Stubby Canyon, there is no evidence of undercutting. The boulders that should be at the base of the canyon are missing.
“These blocks are too big to move by spring flow, and there’s not enough time for the groundwater to have dissolved them away,” Lamb told Caltech’s news operation, “which means that large floods are needed to move them out. To make a canyon, you have to erode the canyon headwall, and you also have to evacuate the material that collapses in.”
Aging the rock samples revealed that the canyon walls of Woody’s Cove and Stubby Canyon had been exposed for the same amount of time, about 46,000 years — not enough time for large rocks to erode.
Amphitheater shaped canyons aren’t common on Earth, though several others can be found near Malad Gorge, but they’re abundant on the surface of Mars, implying that our red neighbor planet has also been shaped by massive catastrophic floods.