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Faults And Earthquakes

As the plates move about, bending and twisting within them produces fractures called faults. Where these reach the surface they can produce scarps—sharp changes in elevation—if movement on the fault had a vertical component. Scarps can be very small, or the size of whole mountain ranges. If a typical mountain range is cut by a fault with large vertical movement, many ridges may be beveled off along the same plane, giving rise to what is often called a faceted mountain range. One classic example is the Grand Teton range in Wyoming.

If the fault movement is horizontal, such as on the San Andreas fault, the grinding up of the rock in the vicinity of the fault may make it susceptible to weathering and erosion. This can result in long, linear valleys such as those in much of southern California. These valleys, if filled with water, become sag ponds, such as the San Andreas Lake. Dry valley floors are often among the flattest terrain, making them prime locations for building municipal facilities. As the reason for their existence has become understood, however, the wisdom of such construction has been called into question.

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