Moraines are geological features created by glaciers, representing the accumulated debris transported by the moving ice. These landforms consist of loose sediment and rock material, collectively known as till, left behind when the ice melts. Lateral moraines are prominent ridges that parallel the sides of mountain valleys. Their presence indicates where a valley glacier once flowed, documenting the history of past ice activity.
Defining Lateral Moraines
A lateral moraine is a long, linear ridge of unsorted glacial sediment deposited along the edges of a valley glacier. This material, or till, is highly variable in composition, containing everything from fine, silt-sized glacial flour and clay to large, angular boulders. The moraine is positioned between the side of the glacier and the adjacent valley wall, forming a distinct embankment.
These features can vary significantly in size, often forming massive debris embankments that run for many kilometers down a valley. Some ancient lateral moraines can reach heights of 100 meters or more. A typical cross-section shows a relatively steep slope facing the former glacier, with a more gradual slope on the side facing the valley wall.
The Process of Formation
The material that forms a lateral moraine originates primarily from the valley walls that flank the glacier. Rock fragments are dislodged from the steep mountain slopes through processes like freeze-thaw weathering, where water repeatedly freezes and expands in cracks. This loosened debris then falls onto the ice surface along the glacier’s margin, often in the form of rockfalls or avalanches.
As the glacier flows down the valley, this accumulated material is carried on the ice surface, known as supraglacial debris. The sheer force of the moving ice also scrapes and plucks material directly from the bedrock of the valley sides, pushing it toward the glacier’s edge. This marginal transport mechanism concentrates the debris into distinct zones along the sides of the ice mass. The linear ridge is created when the ice melts, depositing the carried sediment in a continuous line parallel to the glacier’s former path.
Context within Glacial Landscapes
Lateral moraines are situated exclusively along the flanks of a valley glacier, clearly outlining the maximum elevation and width of the ice. They are distinct from other glacial deposits, serving a specific spatial function within the landscape. The terminal moraine, for instance, is a ridge that forms at the end, or snout, of the glacier, marking the point of its furthest advance.
Another type is the medial moraine, which is found in the middle of a larger glacier. A medial moraine forms when two separate valley glaciers converge. The lateral moraines from their adjacent sides merge to create a single debris stripe running down the center of the combined ice flow.
Geological Significance
Lateral moraines function as archives of past glacial activity and climate conditions. By tracing the crests of abandoned lateral moraines, researchers can accurately determine the maximum past extent and thickness of a glacier. This information helps in reconstructing the paleoclimate, providing data on temperature and precipitation patterns during the time the moraine was formed.
The height of a lateral moraine can indicate the volume of debris transported and the duration for which the glacier maintained a stable position. Scientists can also use techniques like surface exposure dating on the boulders within the moraine to determine when the ice retreated and exposed the feature. Studying these features provides a direct record of how glaciers respond to long-term climate fluctuations.