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.] [Illustration: The peaks in this photograph (a view to the northwest from Marias Pass) are remnants of the overthrust block, which moved eastward. The dividing line between the light-colored rocks and the gray talus slopes beneath them is the Lewis Overthrust Fault.] [Illustration: Glaciation 1 This is how the landscape in this region might have appeared before the onset of the Pleistocene, millions of years ago. Note the stream-eroded, V-shaped valleys. The climate at that time was dry. 2 Glaciers began to form high on the peaks, crept downward, and joined to form larger glaciers. 3 After many centuries of glaciation, tributary glaciers have cut back into the peaks, forming basins called _cirques_. Thick glaciers, moving rapidly and carrying rock fragments, have abraded the main valleys’ floors and sides, widening and deepening the valleys into characteristic U-shapes.] V-shaped Valley Tributary Glacier Unglaciated Peak Headwall Meltwater Stream Nose of Glacier Crevasse [Illustration: 4 In the present landscape, free of all but remnant glaciers, small lakes called _tarns_ occupy many of the cirque basins; and waterfalls plunge into the main valleys from higher, shallower, tributary valleys, called _hanging valleys_. _Alluvial cones_—recent accumulations of rock debris—have begun to build along the valley walls. In the main valley, a _moraine_ (a deposit of rock materials left by the retreating glacier) has formed a dam that holds back a large lake. During all this time, all parts of the terrain not buried under ice and snow have been weathered and eroded by nonglacial forces. Thus the contours of the jagged peaks and sheer cliffs have been softened.]