Uncovering Details of Glacial History by the Marks Left on the Land

Twila Moon
University of Washington, Earth & Space Sciences
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Shortcut URL: https://serc.carleton.edu/69113


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Type: Process


Glaciers can be thought of as large rivers of ice. One of the key elements that separates a glacier from an ice field is that a glacier more actively moves through the landscape, the mechanical properties of the ice allowing it to flow under the force of gravity. This ice motion has a notable affect on the landscape, with ice acting as an important and powerful erosive agent.

Learning about the Past
Glacial erosion features can provide a wealth of information about the past. The most common use of trimlines is identifying previous elevation limits for glacier ice. From these data, scientists can calculate information about what volume of ice was present, where a glacier flowed at specific times in the past, and, in cases areas where glaciers are still present, how much the ice has thinned over time. Glacial striation data is most often used to understand the direction a glacier flowed. Together, these two erosion features might allow a scientist to reconstruct a glacier's ice volume and understand where it moved in the landscape.

Glacial Erosion Features
There are multiple different indicators that glaciers leave on the land due to erosion. These erosional features can be used to study areas where glaciers still exist or unravel the locations and characteristics of glacial ice from hundreds of millions of years ago. Here are two important glacial erosion indicators:

Glacial striation: Scratches created on a bedrock surface by rocks embedded in glacial ice are called glacial striae (Fig. 1). As the glacier ice moves, rocks may be plucked from the glacier bed and become embedded in the ice. The rocks are then dragged over the bed, scratching the bedrock, usually forming multiple, parallel lines. Striation is often best preserved in fine-grained bedrock and can also provide a rough indicator of ice flow direction. The size of striae can range from microscopic to millimeters deep and the characteristic of striae can also depend on topography and the pressure on and orientation of the debris creating the inscription.

Trimlines: A trimline is an erosional mark, usually along a valley wall, that indicates the height of a glacier during the period of erosion (Fig. 2). You can imagine a glacier that flows down through a large, U-shaped valley. The bottom and sides of the glacier are constantly eroding the surface they are in contact with, removing surface vegetation like lichen, and often creating glacial striation or smooth surfaces as they erode away the jagged points of rocks. If the glacier later thins and perhaps recedes, the ice no longer contacts the same height on the valley wall. The mark of the glacier has already been left by the trimline, indicating the previous thickness of the glacier and separating the area of glacier influence (below the trimline) from the area that was not in contact with active glacier ice (above the trimline). Trimlines are not necessarily easy to see in all regions and the topography and type of rock are important factors. Generally, it is easier to identify trimlines on crystalline bedrock that is hard and weather-resistant. These rocks are likely to preserve the trimline features for a longer time than easily erodible surfaces. Continuous rock sections, with few lithologic or structural contacts, are also more likely to preserve trimlines.

Studying Glacial Geomorphology Features
There are a variety of techniques for observing and measuring the geomorphic evidence left by a glacier. Here are three different study methods that span a range of spatial scales, each with its own advantages and disadvantages:

Ground surveys: Ground surveys can provide the most detail for examining glacial erosion marks up close. This may be important for identifying small striae, examining vegetation, or creating very high resolution maps of erosion features. However, with a ground survey, it may be difficult to access areas of interest because of steep slopes and remote locations. Ground surveys can also require lots of work hours to cover an area of interest.

Aerial photography: Using airplanes changes the field information that scientists can collect. Unlike ground surveys, it is possible to image steep slopes and more inaccessible locations. Airplanes can also provide a more efficient method for examining a wide area or study sites in neighboring valleys. Aerial surveys, however, preclude collecting rock samples and may not allow for identification of small erosional features.

Satellite imagery: Studying large regions (~ 102 km scale) may require using satellite data. Different satellite instruments have a range of resolutions. Until recently it was difficult to obtain imagery with resolution better than ~15 m/pixel, which may prove to low a resolution for identifying important geomorphic details. However, new satellite platforms, such as DigitalGlobe and GeoEye, are now reaching resolutions of ~0.5 m/pixel. With this improvement in resolution, satellites may become increasingly popular tools for identifying geomorphological indicators of past glaciation.

Often, scientists use more than one technique so that they can put together a complete picture of what a glacier or glacier system looked like a hundred, a thousand, or many thousands of years ago.

Associated References

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