February 2013
Intermediate to advanced
226 pages
5h 54m
English
Chapter 13
Understanding Change
Analysis of geological evidence suggests that a vast ice sheet covered much of the Northwestern United States and Western Canada 20,000 years ago. Called the Cordilleren Ice Sheet by geologists, it is thought to have covered two and half million square kilometers and melting water from this ice sheet is believed to have created an enormous body of water, Lake Missoula, held back by an ice dam at what is now the Clark Fork Valley. Studies of this area and the surrounding Columbia Gorge and Scablands suggest that periodic rupturing and rebuilding of the ice dam caused enormous floods—scouring out channels, moving boulders the size of buildings, digging out huge potholes, and reforming the landscape of Northern Oregon over the course of an estimated 40 individual flood events.
Imagine glacial Lake Missoula, and great blocks of ice floating on its waters. One day there is calm, smooth water on the lake. A light breeze from the west is warming the waters that are still being fed by the melting ice sheet to the north. The fresh cold water sinks to the bottom and the warmer water rises to the surface. Some of the floating ice has come together and slowly frozen together to make a dam, allowing the level of the lake to rise. However, the warmer surface waters are slowly undermining the ice dam. At some point, a huge section of the dam breaks free and the calm water becomes turbulent as millions of gallons of water are released from the lake. Anything floating ...
