10:40

edge of scablands, gentle rolling hills

At the edge of the scablands are gentle rolling hills. The floods made vast tracks of identical acreage disappear. By civilization's standards, the obliteration of a huge percentage of this rolling landscape is a tragic blow to eastern Washington.

"That material is windblown silt that came from the glaciers that were to the north over many glaciations, probably extending back over many hundreds of thousands of years."

Note the difference in landscape appearance.

11:25

harvesting

The rich soil that produces Washington state's abundant wheat harvest is really dust from rocks ground away by huge ice age glaciers. And as we have seen, much of this glacial dust was torn from the region in the Missoula Floods. But along the margins of the flood, some valleys escaped the destructive force of the raging currents.

Pause tape and ask:

How might a valley escape the water? Watch and see ...

11:50

animation, water stalling to form short-lived lakes, slackwater lakes

Sometimes the racing floodwaters stalled temporarily behind bottlenecks in the landscape -- ridges and other land formations. Huge, but short-lived, lakes would grow as the water backed up behind constrictions. The slackwater lakes dwindled away quickly as the floodwaters crashed onward again.

12:15

Walla Walla River Valley left with markers of the flood

But the lakes left behind evidence of their brief existence -- vast layers of sedimentary deposits.

Here is the Walla Walla Valley near the foot of the Blue Mountains. Backwater from numerous Missoula Floods engulfed the valley -- time after time. And each flood left a unique marker here beneath the gently rolling terrain. "Each one of those layers is a record of an individual flood." A ravine in the Walla Walla Valley reveals ancient history in cross section. "The top of each one of those beds was the surface of the valley for probably several decades. Maybe for something between 50 and 30 years, it was sitting here as a dry surface like this until the next flood came in."

Note the depth of the layers compared to the size of the scientists.

Pause tape and ask:

What caused the layers? Watch the video to find the cause.

13:10

scraping the layers to find two thin layers of ash from St. Helens

Sandwiched between two of the slackwater flood deposits are two very thin layers of volcanic ash.

Mount St. Helens is known to have erupted twice in close succession about 15,000 years ago. Each eruption left a dusting of ash in eastern Washington -- now clearly visible between the flood layers. "It's possible to correlate the same deposit from place to place to place all over part of the region. And if you did not have this ash layer, you'd have no clue." Archaeologists think human beings arrived on the continent some centuries after the last of the great Missoula Floods. There were no eyewitnesses, therefore, and presumably no human victims of the floods. But the clues being unearthed point nevertheless to powerful effects from the floods on the modern world. Missoula Flood layers at Walla Walla, for example, correspond to Missoula Flood layers in Oregon's Willamette Valley some 500 miles away.

Pause tape and ask:

Where did the ash come from?

Why is it significant?

Why is it significant that deposits from Walla Walla, Washington match those in Oregon's Willamette Valley?

14:15

Willamette Valley, deposits 100 feet thick from eastern Washington

That means that much of the fertile soil washed from eastern Washington now rests in the Willamette Valley. In the Willamette Valley, Missoula deposits are 100 feet thick in places.

Time Marks: 0:00-5:00 | 5:00-10:00 | 10:00-15:00 | 15:00-23:20