|
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. 
| 
