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Rock avalanches are often caused by earthquakes or by sudden drops in temperature. In a rock slide, fragments break away from the face of a steep slope and fall down its side. Rock slides are different from rock falls in that they occur on lower slopes, and usually closer to inhabited areas. The worst such event in North America occurred on April 28, 1903. 90 million tons of rock were sent down Turtle Mountain in Alberta, Canada. After a period of warm weather, there was a quick cold spell. Water that had collected in cracks of rock froze, expanded, and shattered rocks. People 25 miles away were able to hear the sound of rock hurtling downhill at 60 miles per hour. Chunks of rock, carried in a huge slide of earth, plowing under the Oldman River. The rock avalanche carried water and parts of the river bottom 400 feet to the opposite side of the valley. When it was all over, the rocks (some the size of railroad cars) were scattered 65 feet deep over one square mile. The rockslide killed 70 people.

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