Geology Reference
In-Depth Information
His colleagues thought this part of the Cascade Range lay submerged beneath the Pacific
Ocean when the boulders arrived, carried by floating ice. Finding no evidence of marine
fossils or ancient beaches, however, Bretz concluded the boulders must have been depos-
ited by fresh water. But what could have generated such an enormous flood?
Each summer he returned to explore farther upstream. After several summers canvassing
the gorge, he shifted north to the scablands. Exploring the strange topography of the area,
Bretz came across dry waterfalls and potholes hundreds of feet above the modern river.
Gigantic gravel bars deposited within coulees (dry valleys) implied deep, fast-flowing wa-
ter. Scour lines that crossed over drainage divides showed that flowing water had over-
topped ridgelines and spilled into adjacent valleys. Streamlined hills rose like islands stick-
ing up more than a hundred feet above the scoured out channelways. Bretz realized the
chaotic landscape had been carved by enormous floods that chewed deep channels through
hundreds of feet of solid basalt. Here, right before his eyes, was the unthinkable.
Ever since Reverend Samuel Parker first described the Grand Coulee as a former channel
of the Columbia River in 1838, explorers and geologists agreed that a glacially diverted
river that ran across the plateau gradually carved the scablands before returning to its nor-
mal valley. But Bretz identified how these now streamless canyons defined a drainage pat-
tern unlike any formed by normal rivers. Here was an interconnected complex of enormous
channels that branched out only to reconnect downstream. Such a network could only form
if water had filled valleys to overflowing and spilled a great flood over their drainage di-
vides. He called this enormous flood the Spokane Flood. But what was the source of all
that water?
Bretz first presented his thoughts on the channeled scablands to the Geological Society
of America in 1923. Focused on describing his field observations, he was careful not to
invoke the taboo of referring to a monstrous flood. He attributed the flows that carved the
valleys to an ice dam across the Columbia River that forced water to spill out across the
scablands. Over successive summers Bretz became increasingly confident that the scab-
lands were not just the gradually produced work of a diverted river.
He recognized that 100-foot-high piles of gravel on the canyon floors were built by even
deeper flows and that the hanging valleys that drained over dry waterfalls were not the
product of normal stream erosion. These features were carved by a process that shut off
