Lewis Overthrust Fault
The Lewis Overthrust of Waterton/ Glacier provides scientists with insight about the massive dynamics of geologic processes that are going on today in other parts of the world, such as the Andes and the Himalaya Mountains. Because of the high degree of preservation of the original rock characteristics, the recent glacial sculpturing of the rocks, and the access by roads and trails, this major geologic structure in Waterton/Glacier Park is available for study by scientists from around the world.
The Lewis Overthrust began 170 million years ago, when a collision of the Earth s crustal plates elevated numerous mountain chains and formed the ancestral Rocky Mountains. Ever-increasing stresses near the end of this great event shoved a huge rock wedge, several miles thick and several hundred miles wide, eastward more than 50 miles. Large masses of relatively stronger rocks were shoved over softer and more easily deformed rocks. Erosion stripped away the upper part of the original rock wedge and exposed the rocks and structures visible in the park today. Rarely have rocks of such ancient age been thrust over rocks that are so much younger. The overlying Proterozoic rocks are over 1,500 million years older than the underlying Cretaceous age rocks.
Thus, the Lewis Overthrust is significant as a structural feature, for the extent of lateral displacement (up to 80 kilometers), and because it has functioned to expose ancient sediments possessing an unparalleled degree of preservation.
Of particular scenic and geologic note is Chief Mountain, a spectacular monolith towering above the prairie along the eastern margin of Waterton/ Glacier. Chief Mountain is an erosionally isolated remnant of the eastern edge of the upper plate of the Lewis Overthrust -- a feature known as a Klippen ranking with the Matterhorn as an example of this structural and erosional phenomenon.
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Proterozoic Sedimentary Rocks
Most of the rocks exposed in the park are sedimentary rocks of Proterozoic age, which were deposited from 1,600 to 800 million years ago. Rocks of that age in other parts of the world have been greatly altered by mountain building processes and no longer exhibit their original characteristics. These virtually unaltered Proterozoic rocks of Waterton/Glacier are unique in that they have preserved the subtle features of sedimentation such as ripple marks, mud cracks, salt-crystal casts, raindrop impressions, oolites, six species of fossil algae, mudchip breccias, and many other bedding characteristics.
These Proterozoic sedimentary rocks, while outcropping over an area extending from southern Montana to southern British Columbia, are most impressively exposed in Waterton/ Glacier. Due to the extreme relief and unexcelled exposures, over 2,100 meters of stratigraphic thickness is exposed to scientific examination. These features plus their chemical characteristics make the Proterozoic sediments of Glacier and Waterton National Parks unique for studying the physical and chemical conditions that existed on the Earth over a billion years ago. Such information is of great importance to scientists in understanding the stability or changes of the Earth s climates through geologic time. The recent glacial carving of these rocks has left them unusually fresh and beautifully exposed.
Several of the sedimentary rock layers described above, contain fossils called stromatolites. They were colonial organisms of blue-green algae that lived in warm shallow seas marginal to ancient lands. Six species representing three genera of stromatolites are preserved in the ancient sediments of the park. Because of the high degree of preservation of the rocks in which these fossils occur, the stromatolites of Waterton/Glacier contain such detail as to make them unique. Paleontologists from around the world come to Waterton/Glacier to study these fossils because of their preservation, diversity, and antiquity. These fossils are a major source of information concerning the physical and chemical conditions on the Earth for a time period of about 800 million years, at a time over a billion years ago. A professional geologist for the United States Geological Survey recently compared these ancient rocks and fossils of Waterton/Glacier to the rare book section of the world’s geological library.
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What is a Glacier?
A glacier forms when more snow falls each winter than melts the next summer. The accumulation of snow above presses down on the layers below, and compacts them into ice. Ice near the surface of the glacier is often hard and brittle but, due to the pressure of ice above, the ice near the bottom of the glacier becomes flexible. This flexible layer allows the ice to move. Depending on the amount of ice, the angle of the mountainside, and the pull of gravity, the ice may start to move downhill. Once this mass of snow and ice begins to move, it is called a glacier.
Glaciers Past and Present
The glaciers in Glacier National Park today are all geologically new having formed in the last few thousand years. Presently, all the glaciers in the park are shrinking. More snow melts each summer than accumulates each winter. As the climate changed over the last two million years, glaciers formed and melted away several times. What will happen to today's glaciers if the climate becomes colder, wetter, or warmer?
Geologists theorize that about 20,000 years ago the climate became cooler and/or wetter. This allowed for the formation of huge glaciers that filled the valleys with thousands of feet of ice. Imagine the valleys of Glacier National Park filled with ice, and just the tops of the highest peaks sticking out. These giant rivers of ice sculpted the mountains and valleys into their present appearance. Today's glaciers are carving at the mountains as well. Although smaller, they work in the same way as the larger glaciers of the past, and teach us about Glacier National Park's geologic history.
Sculpting the Land
As the ice moves, it plucks rock and debris from the sides and bottom of the valleys. Rocks falling on the glacier from above mix with the glacial ice as well. A glacier is filled with rock and gravel. Over long periods of time the sandpaper-like quality of the moving ice scours and reshapes the land into, broad U-shaped valleys, sharp peaks, and lake filled basins. Massive ancient glaciers grinding over he bedrock below produced the spectacular landforms seen today.
The Park is filled with horns, cirques, arêtes, hanging valleys, and moraines; landforms given special names because they were produced by the action of glaciers.
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A horn is a steep mountain peak caused by several glaciers carving different sides of the same mountain. Mt. Reynolds at Logan Pass is a good example of a horn.
Iceberg Cirque. NPS Photo
A cirque is a large bowl formed at the head of a glacier. Often as the ice melts away a small lake will form in the depression gouged by the glacier. Avalanche, Iceberg, and Gunsight are all excellent examples of cirque lakes.
An arête (French for fish-bone) forms when two glaciers work on opposite sides of the same wall, leaving a long narrow ridge. One of Glacier National Parks more prominent features, the Garden Wall, is an arête separating the Lake McDonald Valley from the Many Glacier Valley.
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Hanging Valleys are found throughout the park. As large glaciers scoured the main valleys, tributary glaciers worked the smaller side canyons. Unable to cut as deep as the valley glaciers, they left behind small valleys high up on the mountainsides. Frequently hanging valleys have waterfalls cascading out of their mouths into the valleys below. Birdwoman Falls, seen from the Going-to-the-Sun Road, plummets from a hanging valley on Mt. Oberlin.
Moraines form at the sides and front of a glacier. In a glacier there is always a flow of ice from the head to the toe. This conveyor belt like flow brings with it the rock and debris trapped in the ice. As it reaches the sides or front and the ice melts, this trapped material is released forming large piles. These piles of glacially transported material are called moraines. Moraines from the present glaciers are visible as mounds of rock and gravel along the sides and front of the ice. Plants soon colonize this new soil. Forests and meadows cover many ancient moraines making them harder to spot.
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Information provided by the National Park Service