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Definition: moraines from Environmental History and Global Change: A Dictionary of Environmental History

Ridges and mounds of debris deposited by active glaciers or let down on to the surface by the melting of ‘dead’ ice. Moraines may be deposited at the margins of glaciers (lateral moraines) and especially at their snouts (terminal moraines). The finest material (rock flour) is washed out by glacial meltwater leaving coarser material up to the size of boulders behind. Moraines can be dated using radiocarbon, lichenometry and other methods to provide chronologies of periods of glacier advances and retreats.


Summary Article: MORAINES from Encyclopedia of Earth Sciences Series: Encyclopedia of Paleoclimatology and Ancient Environments

A moraine is a glacial landform created by the deposition or deformation of sediment by glacier ice. Many different types of moraine exist, reflecting the many different processes by which glaciers deposit and deform sediment and the many locations and environments within the glacier system where deposition can occur. Moraines are composed of till, which is also highly variable, as its characteristics depend on the characteristics of the debris supplied by the glacier as well as on the processes and environment of glacial deposition.

The morphology and sedimentology of moraines can be used to reconstruct the characteristics of former glaciers. Their distribution reflects the geography of former glaciers and glacial process environments. Sediment characteristics reflect the source location of the debris: supraglacial debris is characteristically angular, while basally derived debris is typically facetted, subrounded and striated. Structures within moraines can reveal seasonal and long term variations in processes of sedimentation.

Moraines are classified both genetically according to the process by which they are created and geographically according to their position within the glacier system. The principal processes by which moraines are created are the release of debris from ice by meltout and the deformation of proglacial or subglacial sediments by ice motion. Supraglacial moraines include lateral moraines, medial moraines and inner moraines. Subglacial moraines occur parallel or perpendicular to ice flow or in irregular patterns. Ice marginal moraines occur around the edges of glaciers and are defined by their position as either lateral or frontal moraines. Moraines marking the maximum extent of a glacial advance are referred to as terminal moraines. Moraines deposited at successive positions of the margin during a period of progressive retreat are recessional moraines. Marginal moraines at existing glaciers are typically ridges of sediment resting partially on the edge of the glacier. Upon deglaciation, ice-cored moraines lose their ice support and therefore tend to shrink in size and may become structurally unstable. Marginal moraines may be several tens of meters in height, tens or hundreds of meters across, and may stretch for hundreds of kilometers around the margins of large ice sheets.

Moraines provide long-term storage within the glacier sediment transfer system, and a supply of debris to the proglacial zone. Sediment flux within glaciated basins is very sensitive to the position of glaciers relative to their moraines. When glaciers lie behind marginal moraines the bulk of sediment produced at the margin can go into storage in the moraine belt and not reach the proglacial region. When glaciers have no marginal moraines, sediment passes directly into the proglacial system. Moraines can also focus meltwater discharge, localizing fluvial processes and causing moraine-dammed lakes. These lakes are potentially unstable and pose a serious threat of catastrophic flooding. Bennett and Glasser (1996) and Benn and Evans (1998) provide a useful review of this topic.

Cross-references

Glacial Geomorphology

Glacial Sediments

Tills & Tillites

Bibliography
  • Benn, D. I.; Evans, D. J.A., 1998. Glaciers and Glaciation. Arnold London UK, 734pp.
  • Bennett, M. R.; Glasser, N. F., 1996. Glacial Geology. Wiley Chichester UK, 364pp.
  • Peter G. Knight
    © 2009 Springer Science + Business Media B.V.

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