Had town of Missoula, Montana, existed 1000’s of years in the past, it might have been below water.

Over the last Ice Age, a sheet of ice 20 miles vast received caught within the Idaho panhandle and blocked the Clark Fork River, creating glacial Lake Missoula. At its highest, the water degree reached 4,250 ft above sea degree—over 1,000 ft above the current metropolis’s altitude. The ice sheet in the end gave solution to the strain of the water, and glacial Lake Missoula drained catastrophically.

It’s estimated that the most important flood discharge reached 386 million cubic ft per second. At that fee, it took the lake only some days to empty, with its waters ultimately reaching the Pacific Ocean.

The scariest factor isn’t the size of this occasion—it is that floods of this measurement occurred a number of occasions. Hundreds of years after that first flood, scientists lastly pieced collectively whether or not it was a one-time deal by trying to the grime for solutions.

Indicators of the previous

Within the fall of 1969, Wealthy Chambers drove southwest of Missoula alongside I-90 together with his undergraduate adviser. They pulled over to the facet of the street to take a look at a wall that ran 80 ft into the air. It was zebra-striped, with layers of darkish and light-weight sediments working horizontally up the slope.

Missoula is the second-biggest metropolis in Montana, with a inhabitants of about 75,000, and it sits in a mountain valley. The College of Montana is thought extra for its forestry and regulation faculties and fewer for the large boulders sitting round campus or the traces on two mountains—seen from about wherever on campus—which might be remnants of the lake that after drowned the valley.

Chambers devoted his undergraduate and grasp’s work to glacial Lake Missoula, which shaped behind the Cordilleran Ice Sheet between 14,000 and 21,000 years in the past. The glacial lake would cowl virtually 3,000 sq. miles and maintain as a lot water as Lake Ontario and Lake Erie mixed.

Chambers’ adviser was David Alt, a well-renowned historian of glacial Lake Missoula. One thing Alt wasn’t as conversant in, although, was the sediments left behind after the flood drained the lake—the kind of sediments that he and Chambers discovered themselves off I-90.

“These are Lake Missoula sediments,” Alt stated to Chambers as they stared on the zebra wall. “And no one’s them intimately.”

If there was to be something new uncovered concerning the lake’s historical past, it might come from sediments like these. And there was some urgency in uncovering it—within the Seventies, there was a giant back-and-forth brewing within the scientific neighborhood concerning what number of occasions the lake might have drained and refilled.

Land of many lakes

Chambers began classifying Lake Missoula’s sediments and seen two scales of zebra striping. On the massive scale, he discovered about 40 alternating gentle and darkish soil sequences as much as a number of meters thick. These cycles, known as rhythmites in geology-speak, are deposits the place the sunshine layers are made up of high quality sand and silt deposited by rivers within the early phases of a lake’s filling, whereas the darkish layers are made up of silt and clay that collect on the underside of stuffed lakes.

Chambers then seen that the darkish layers had their very own zebra stripes. The stripes-within-stripes at this scale are known as varves, they usually doubtless represented annual layers of sediment stacked on high of one another. The varves instructed geologists concerning the period of time it took for the lake to fill. Assuming the varves represented annual stackings, Chambers stated it took solely a median of about 50 years for the lake to refill. Though the lake’s depth decreased with every filling, that’s nonetheless a wild quantity of water. At its peak, it was greater than 500 cubic miles of water, about half the amount held in Lake Michigan.

Little by little, Chambers and Alt pieced collectively an image of the realm’s historical past. In a paper that Chambers and Alt co-authored, they instructed a number of many years might need handed between every lake draining and filling; in a unique paper, Chambers concluded there was no proof that the glacial lake had drained utterly every time. He later argued that the final a number of lake drainages had been much less intense, which saved the floods from washing away deposits just like the one seen on the roadcut on I-90.

That piece of land off the freeway comprises a minimum of 800 years of historical past—and presumably extra. It’s unclear how a lot sediment might have been faraway from the document by subsequent floods. Every draining of glacial Lake Missoula may have taken with it proof of earlier floods.