Title Bar: Illustrated Glossary of Alpine Glacial 
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    Medial moraine

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A ridge of unconsolidated material formed on top of and in the middle of an existing glacier.

When two alpine glaciers flow together their lateral moraines join to form a medial moraine. Medial moraines can also form downstream from rock outcrops extending up through the surface of alpine glaciers. As glaciers melt and the moraine sediment is deposited on the landscape, meltwater may redistribute the sediment and cause the ridge to lose its form. As a result, you may not be able to recognize the sediment as part of a former medial moraine once the glacier is gone.

The row of rocks and debris running from the left to the right side of the photo is an example of a medial moraine. This medial moraine sits on top of the Mendenhall Glacier in Alaska.

Photo: medial moraine

Photo: L. Freeman. Used with permission.

This photo shows two medial moraines (blue arrows) as seen from the air. The black arrows point to lateral moraines that have formed along the sides of the valley wall. Where the two main glaciers shown in the photo flow together, their lateral moraines join to form a medial moraine. The medial moraine on the right formed in the same way, however the glaciers and lateral moraines that flowed together to form this moraine are out of the photo.

Photo: medial moraine

Photo: A. Harris. Used with permission.

This photo shows a medial moraine forming down-glacier from a mountain peak that extends above the surface of the glacier.

A small glacier flows in from the left side of the photo and where this small glacier once joined the main glacier in the photo you would expect to see a medial moraine, and there is a well-defined ridge of till in that location. There is no longer any ice to the left of this moraine, however, because the small glacier has retreated.

Photo: medial moraine

This photo shows an example of a medial moraine that is still visible after the glaciers that formed it have melted away. There are also some very good lateral moraines in the photo. Meltwater streams are currently modifying the appearance of the glacial till and the moraines.

Photo: Medial moraine

Photo: B. Molnia. U.S. Geological Survey Alaskan Glaciers, image dsc00035. <http://www.usgs.gov/features/glaciers2.html> Accessed July, 2010. Photo: D. Becker. 2008. USGS Multimedia Gallery, photo BeckerDSC_4815.jpg. <http://gallery.usgs.gov/> Accessed July 2010.
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All commercial rights reserved. Copyright 2010 by Karen A. Lemke. Earlier copyright 2002 by Karen A. Lemke, and 1998 by Karen A. Lemke and Linda Freeman.
Last updated July 2010 KAL.