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.