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One of the Earth's strangest geological riddles is the evidence for
a huge catastrophe that struck eastern Washington State thousands of
years ago. It took scientists decades to figure out that a colossal
flood had carved out bizarre landscape features strewn across
thousands of square miles. On "Mystery of the Megaflood," NOVA gets
to the bottom of what created this compelling detective story.
"Mystery of the Megaflood" features a dogged geologist sticking to
his bold theory for decades despite virtual professional banishment.
Eventually, other geologists joined his cause and filled in the
intricate details, which NOVA recreates in stunning computer
animation to show what may be one of the most spectacular series of
events ever to occur on our planet.
The so-called "scablands" are a vast region of weird terrain 200
miles east of Seattle, including gorges hundreds of feet deep,
enormous pits, huge boulders scattered as if dropped by giants,
undulating hills that look like huge ripples, strange layers of silt
and ash, and a "waterfall" five times wider than Niagara, but
without any water (see
Explore the Scablands). The name "scablands" perfectly suits the scarred and wounded
landscape, which baffled most geologists throughout the 19th century
and much of the 20th. To them, no plausible explanation fit all the
facts.
For example, there is no large river cutting through the scablands
that could have carved the features over millions of years—as
the Colorado River did in the Grand Canyon. Nor is there evidence
that the area was buried beneath glaciers that produced extensive
erosion—as occurred in large sections of the American and
Canadian Rockies.
But during the 1920s a geologist named J Harlen Bretz outlined a
startling hypothesis. His fieldwork convinced him that the scablands
were not the result of slow geological weathering, but of an
enormous catastrophe that had taken place almost overnight when a
titanic flood engulfed the region. Many of his colleagues ridiculed
the idea, especially because it smacked of "catastrophism," a
discredited view that Earth had been shaped by sudden cataclysms
rather than by slow evolutionary change.
Bretz was unable to say where all the water had come from, but a
colleague named Joseph Thomas Pardee was certain that the answer lay
in the region around Missoula, Montana, where the surrounding
mountains held evidence that an enormous lake had once filled the
basin (see
Ice Age Lake). This
lake formed when a glacier plugged the valley below Missoula during
the last ice age, creating a natural dam and eventually a body of
water that was half the size of Lake Michigan. If that dam suddenly
burst, the path of the released water would rush directly over the
scablands, scouring exactly the kinds of features that are observed
there today.
All that was needed was a natural mechanism to breach the dam and
release Lake Missoula—no easy feat since the glacier was
probably thousands of feet thick. Such a mechanism was finally
discovered after a much smaller glacial dam burst in Iceland in
1996, causing incredible devastation in the valley below.
NOVA takes viewers on a virtual tour inside a glacier to see how
tremendous pressure creates tunnels of supercooled water that, over
time, fatally weaken the structure of an ice dam, causing it to
fail. The current plot twist to the scablands story is that a deluge
happened not once, but repeatedly, as ice dams reformed and the
glacial lake refilled, only to empty again and again onto the
scarred terrain of what is now eastern Washington.
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