Geoscience Reference
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called the Laurentide ice sheet—that blanketed Canada and the north-
ern United States at the end of the last ice age. As the earth warmed, the
Laurentide ice sheet melted. h e waters that had been locked in the ice were
released to make their way back to their place of origin—the ocean. In North
America, these waters took several routes: east through the St. Lawrence
Seaway to the North Atlantic; south through the Mississippi River to the
Gulf of Mexico; and west through the Columbia River to the Pacii c Ocean.
One of the largest of the glacial lakes was named Missoula, and it covered
much of northern Montana. Even as Bretz was mapping giant l ood features,
a glacial geologist named Joseph h omas Pardee was studying the sediments
deposited at this lake's bottom. As Macdougall's topic also describes, Pardee's
work revealed that Lake Missoula was once the size of Lake Michigan
but even deeper: ancient lake shorelines mapped in the hills surrounding
Lake Missoula revealed that its depth was, in places, 2,000 feet. Linear hills
45 feet high and separated by over 400 feet were enormous ripple marks
that formed just west of Lake Missoula under the huge volume of l oodwater
draining the lake.
Putting together all the evidence, Bretz hypothesized that an ice dam once
held back the lake, and, when that dam melted, the water trapped behind it
burst forth in a catastrophic l ood. h e sound of the bursting dam would
have been deafening as it rumbled for hundreds of miles ahead of a wall of
water 500 feet high pouring across the Columbia Plateau toward the Pacii c
Ocean. It would have swept up everything in its path, a brown and churn-
ing wave of water, soil, vegetation, and l oating icebergs still embedded with
boulders. h e discharge rate of this massive l ood was likely over 200 times
that of the largest historical l oods on the Mississippi River.
h is terrifying l ood was probably the largest but not the only one that
occurred in the Northwest as the earth thawed from the last ice age. Evidence
for multiple l ood events comes from the layers of sediment accumulated
in the Scablands. h e scoured debris and sediments in these l oodwaters
were eventually carried out to the Pacii c Ocean to a i nal resting place over
600 miles from the Oregon coast, where they were recently cored and
recognized as further evidence for the megal oods.
As we have seen, a number of factors inl uenced climate over dif erent
timescales. h e earth's climate is af ected by many powerful processes,
including changes in its orbit, changes in solar output, volcanic eruptions,
