How is a geologist like a detective?
A geologist uses evidence left by events to reconstruct a geological history. Where should we go to study the tracks glaciers have left behind but Glacier National Park in Montana? The glaciers in the park have melted back a great deal in the past century, continuing a trend that began at the end of the ice ages. The features left behind by the glaciers are visible for everyone to see.
Glaciers erode the underlying rock by abrasion and plucking. Glacial meltwater seeps into cracks of the underlying rock. When the water freezes, it pushes pieces of rock outward. The rock is then plucked out and carried away by the flowing ice of the moving glacier (Figure below). With the weight of the ice over them, these rocks can scratch deeply into the underlying bedrock, making long, parallel grooves in the bedrock, called glacial striations.
Glacial striations point the direction a glacier has gone.
Mountain glaciers leave behind unique erosional features. When a glacier cuts through a V-shaped river valley, the glacier plucks rocks from the sides and bottom. This widens the valley and steepens the walls, making a U-shaped valley (Figure below).
A U-shaped valley in Glacier National Park.
Smaller tributary glaciers, like tributary streams, flow into the main glacier in their own shallower U-shaped valleys. A hanging valley forms where the main glacier cuts off a tributary glacier and creates a cliff. Streams plunge over the cliff to create waterfalls (Figure below).
Yosemite Valley is known for waterfalls that plunge from hanging valleys.
(a) A bowl-shaped cirque in Glacier National Park was carved by glaciers. (b) A high altitude lake, called a tarn, forms from meltwater trapped in the cirque. (c) Several cirques from glaciers flowing in different directions from a mountain peak, leave behind a sharp sided horn, like the Matterhorn in Switzerland. (d) When glaciers move down opposite sides of a mountain, a sharp edged ridge, called an arête, forms between them.
Snowmelt and melting glaciers combine to create a fast moving stream at Glacier National Park.
Depositional Features of Glaciers
As glaciers flow, mechanical weathering loosens rock on the valley walls, which falls as debris on the glacier. Glaciers can carry rock of any size, from giant boulders to silt (Figure below). These rocks can be carried for many kilometers for many years.
Rocks carried by a glacier are eventually dropped. These glacial erratics are noticeable because they are a different rock type from the surrounding bedrock.
A large boulder dropped by a glacier is a glacial erratic.
Melting glaciers deposit all the big and small bits of rocky material they are carrying in a pile. These unsorted deposits of rock are called glacial till. Glacial till is found in different types of deposits. Linear rock deposits are called moraines. Geologists study moraines to figure out how far glaciers extended and how long it took them to melt away.
Moraines are named by their location relative to the glacier:
- Lateral moraines form at the edges of the glacier as material drops onto the glacier from erosion of the valley walls.
- Medial moraines form where the lateral moraines of two tributary glaciers join together in the middle of a larger glacier (Figure below).
The long, dark lines on a glacier in Alaska are medial and lateral moraines.
- Ground moraines forms from sediments that were beneath the glacier and left behind after the glacier melts. Ground moraine sediments contribute to the fertile transported soils in many regions.
- Terminal moraines are long ridges of till left at the furthest point the glacier reached.
- End moraines are deposited where the glacier stopped for a long enough period to create a rocky ridge as it retreated. Long Island in New York is formed by two end moraines.
(a) An esker is a winding ridge of sand and gravel deposited under a glacier by a stream of meltwater. (b) A drumlin is an asymmetrical hill made of sediments that points in the direction the ice moved. Usually drumlins are found in groups called drumlin fields.
- Try to pick out some of the glacial features seen in this Glacier National Park video: http://www.visitmt.com/national_parks/glacier/video_series/part_3.htm.
Several types of stratified deposits form in glacial regions but are not formed directly by the ice. Varves form where lakes are covered by ice in the winter. Dark, fine-grained clays sink to the bottom in winter, but melting ice in spring brings running water that deposits lighter colored sands. Each alternating dark/light layer represents one year of deposits.
- Glaciers have more force than any of the other erosional agents because of their incredible mass. As a result, they can erode the landscape. Glacial features in alpine areas are beautiful.
- Glaciers dump material, leaving clues for scientists as to where the glacier went. Glacial moraines outline a glacier's extent.
- Varves form in lakes covered by ice. Varves are useful to scientists for understanding climate.
Use this resource to answer the questions that follow.
- What is the main erosive process of a glacier and how does it erode material?
- How does abrasion happen in a glacier?
- What causes striations?
- How does freezing and thawing promote erosion?
- How does a glacier carry material so far?
- How can glacial striations be used to indicate the direction a glacier moved? What is the process that creates striations?
- How do glaciers modify mountain terrain? What are some of the features they create?
- What information can scientists get from varves and how do they get it?