The landscapes of Glacier National Park reveal three intervals of geologic history.

1) Layers of sediment were deposited in a body of water, either a large embayment of the ocean or a very large lake. This took place during the Proterozoic Era, from about 1.5 billion to about 900 million years ago. During this time, a body of molten magma rose up from the Earth's mantle and intruded between two layers of sediment. It solidified to form the Purcell sill that shows up in the picture as a dark horizontal band in the rock wall at the top right, with white stripes above it and below it and a patch of snow obscuring it at the right.

2) In the Late Cretaceous and Early Tertiary periods (about 70 to 40 million years ago), the layers of rock were thrust upward and toward the east along huge thrust faults.

3) During the Pleistocene Epoch (2 million to 10 thousand years ago), glaciers accumulated into ice caps on the crest of the uplifted mountain range. The flowing ice carved out the cirques and deep u-shaped valleys, matterhorn- type peaks and jagged ridges that give the park its majestic alpine scenery. Then the glaciers retreated and melted away, except for a few small glaciers that still exist in the park today. The remaining glaciers are retreating and may disappear completely within a few decades.

The photograph shows Lake Grinnell and the upper Grinnell Valley. The entire scene was once beneath the ice cap that covered most of the crest of Glacier National Park. Now all that remains of glacial ice in this valley is the retreating Grinnell Glacier, which is hidden behind the dark peak that dominates the center of this view.

The Purcell Sill

The Purcell Sill is visible in this view of the wall of the cirque of Grinnel Glacier. The sill is the dark stripe that runs left to right across the middle of the picture, with bands of white rock below it and above it. This sill is an intrusion, formed when hot, molten magma forced its way between layers of sedimentary rock, then cooled and crystallized into solid rock of the type called diabase. Diabase can be thought of as coarse-grained basalt, or fine-grained gabbro. The age of the Purcell Sill has been determined by radiometric isotopes to be approximately 1.2 billion years, which puts it in the Proterozoic Eon. Select the image to see a larger view. Use your browser' s back button to return to this page.

The jagged ridge that forms the skyline is an arête, a ridge formed by the erosive action of glaciers carving away rock on both sides of the ridge.

The large patch of snow on the right, which covers the Purcell Sill, is called The Salamander. Beneath the white snow left over from the previous winter, the near-surface of the Salamander is made of firn and, down deeper, denser ice. Firn is the grainy ice formed from snow after a year or two of partial thawing, re-freezing, and recrystallization. The Salamander is a remnant of the larger glacier that once filled the whole valley, which was most recently at its maximum approximately 18,000 years ago, near the end of the Pleistocene Epoch.

The Grinnell Formation

This image shows two types of sedimentary rock in the Grinnell Formation in Glacier National Park. The pocketknife provides a scale for the photo. The red, fine-grained rock is shale. The gray-white rock is sandstone. The shale started out as mud that accumulated at the bottom of a body of stagnant water. The sandstone formed from sand that was carried across the mud layers by water that flowed energetically, possibly during a storm. The flowing sand and water ripped up pieces of mud, which became included within the sand. These are called rip-up clasts. The rip-up clasts are the angular red pieces of shale in the upper half of the picture, surrounded by gray-white sandstone.Select the image to see a larger view. Use your browser' s back button to return to this page.

Hanging Valleys

Three hanging valleys appear on the left of this picture. Each hanging valley once contained a glacier that flowed out of the scoop-shaped cirque it had eroded into the bedrock. Each of these valley glaciers joined into the larger trunk glacier. The trunk glacier was the deeper, much longer valley glacier that was fed by several tributary glaciers. These types of erosional features, with steep-walled cirques, hanging valleys and u-shaped glacial troughs, are hallmarks of the extensive alpine glaciation that once covered much of what is now Glacier National Park. Similar evidence of the recent presence of large glaciers is found in many other mountain ranges of the Pacific Northwest, including the Olympic Mountains, the Cascades, the Wallowa Mountains, and other parts of the Rocky Mountains. But nowhere else in the Pacific Northwest was the alpine glaciation so deep and widespread as it was in the Glacier National Park area. Select the image to see a larger view. Use your browser's back button to return to this page.

Mudcracks

This image shows mudcracks on a bed of siltstone in the Grinnel Formation. These cracks, also known as syneresis cracks, form when muddy sediments dry up and the clay that is in the mud shrinks. These features, preserved from the Proterozoic Era over a billion years ago, show that the sediments were deposited in a body of water that occasionally dried up or retreated from the land. Select the image to see a larger view. Use your browser's back button to return to this page.

Glossary terms that appear on this page: magma; sill; thrust fault; basalt; gabbro; arête; shale; sandstone; rip-up clasts; hanging valley; siltstone