Features of Glacial Topography

Glacial Erosion and Deposition Create Distinct Landforms

Oct 23, 2009

Glaciers leave signs of their presence on the surrounding landscape through erosion and through the sediment deposits they create.

Glaciers are rivers of ice that form through the accumulation of snow over time. They are always on the move, and Earth’s global temperature has experienced many fluctuations throughout its history. For these reasons, glacial environments are in constant flux. Often, it is not the presence of glaciers themselves, but rather the features they create that allow geologists to determine the glacial history of a region. Distinct glacial topography is formed in two main ways: the erosive action of the glacier as it moves across the landscape and sedimentary deposits that are left behind as the glacier retreats.

Features Formed By Glacier Erosion

In Earth: Portrait of a Planet (W.W. Norton and Company, 2001), Stephen Marshak describes several topographical features created by glacial erosion:

A cirque is a bowl-shaped depression located on the side of a mountain. Cirques form at the head of a glacier where freezing and thawing over time fractures the underlying rock, which is then incorporated into the glacier. Lakes that form in these depressions are called tarns.

Cirques are related to two other glacial features. An arête is a very narrow ridge of rock that separates two adjacent cirques. A horn is a pointed mountain peak surrounded by several cirques.

Mountain glaciation forms many types of valleys. U-shaped valleys, characterized by their curved bottoms and steep sides, form when glaciers travel through previously existing drainages. Hanging valleys form where tributary glaciers flow into an area’s main, or trunk, glacier. Due to the size and weight of the trunk glacier, the valley it forms is much deeper than that formed by the tributary glacier. When the glaciers recede, the floor of the hanging valley is high above the floor of the trunk glacier valley. Fjords are glacial valleys that have been submerged.

The formation of glacial valleys also leads to the formation of truncated spurs. Spurs are ridges between pre-existing drainage valleys. When glaciers move through a valley, the spur’s tapered end is eroded away, leaving behind a blunted landform, or truncated spur.

Features created during continental glaciation depend on the pre-glacial landscape. In areas that were already flat and low-lying, glaciers grind across the bedrock, leaving distinct polished, striated surfaces. Glacial striations are typified by long gouges, grooves, or scratches where particles embedded in the glacier have ground against the surface of the bedrock. In areas that were hilly prior to glaciation, valleys are deepened and hills are smoothed and rounded by the action of the ice. After the glaciers retreat, hills are elongate in the direction of flow and have an asymmetric shape. These hills are called roche moutonnée.

Glacial Deposits Create Unique Land Features

According to Marshak, there are also several depositional features that are clues to a region’s glacial history:

There are several types of moraine deposits. An end moraine forms when the terminus of the glacier stays in one place for a prolonged time, creating a low, sinuous ridge. A terminal moraine is an end moraine that indicates the farthest limit of glaciation. A recessional moraine forms when a glacier stalls for long periods of time as it recedes. If a glacier flows over a moraine it has previously deposited, the ridge is shaped into an elongate hill called a drumlin. Erratics are large boulders that are dropped by a glacier.

Outwash plains are broad areas of gravel and sandbars where braided meltwater streams deposit till, or unsorted glacial sediments. Braided streams form due to the large amounts of sediment present in the outwash near the toe of a glacier. There is so much sediment, in fact, that meltwater tunnels at the base of a glacier can fill with it. When the glacier melts, this leaves behind ridges of till called eskers.

Kettle holes are circular depressions that form when blocks of ice calve off the bottom of the glacier and become buried by till. Eventually, the ice melts and the overlying till collapses into the empty space. Often kettle holes fill with water, creating lakes. An area with many kettle holes and lakes separated by rounded hills of till sediment is said to have knob-and-kettle topography.

Topography Leaves Clues to Glacial Past

Many of the features described by Marshak can be located in the vicinity of an active glacier. However, in some locations, glaciers have long since melted and disappeared. The features of glacial topography allow scientists to piece together the region’s geologic and climatic past.

The copyright of the article Features of Glacial Topography in Geography is owned by Dianne Turgeon. Permission to republish Features of Glacial Topography in print or online must be granted by the author in writing.