File:Lewis overthrust fault nh10f.jpg

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schematic illustration of the Lewis overthrust fault in northern Montana, USA and Southern Alberta, Canada. The fault can be seen best in Glacier National Park, USA and Waterton Lakes National Park, Canada.

original caption: Overthrust Mountains

1 A hypothetical block of the Earth's crust in the region of Glacier National Park as it existed more than 60 million years ago. The two layers shown actually represent many strata of sedimentary rocks.

2 Lateral pressure begins to force the rock layers to buckle.

3 A large fold has been created, forcing the rock strata to double over and overturning some layers. A break, or fault, is forming at the plane of greatest stress.

4 The break has been completed and the strata west of the fault have slid eastward, up and over the rocks east of the fault.

5 The Glacier landscape today. Throughout the millions of years during which the folding, faulting, and overthrusting have been taking place, the process of erosion has continued; a thousand meters of stratified rocks have been worn away, so that only a remnant of the overthrust layers can be seen today. Because Glacier's eastern slope represents the eroded face of the overthrust block, the mountain range rises precipitously from the prairie, with no foothills breaking the abrupt transition from open prairie to mountain valley.
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Source National Park Service http://www.nps.gov/history/history/online_books/natural/10/images/nh10f.jpg - used in the online edition of the book "MANY-STORIED MOUNTAINS - The Life of Glacier National Park" by Greg Beaumont, first published 1978 and online at http://www.nps.gov/history/history/online_books/natural/10/nh10b1.htm
Author Greg Beaumont, National Park Service
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Public domain This image or media file contains material based on a work of a National Park Service employee, created as part of that person's official duties. As a work of the U.S. federal government, such work is in the public domain in the United States. See the NPS website and NPS copyright policy for more information.

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