Legacy of the Ice - Moraines
Rock fragments, sand and clay eroded and picked up from the ground beneath a glacier are carried forward as a carpet of debris at the base of the ice. Some of this material (till or boulder clay) is deposited as a layer beneath the glacier (till sheet), but some is transported to the glacier edge where it accumulates as mounds or ridges (moraines). Large moraines are formed at times when the ice front remains in the same place for lengthy periods. They are, therefore, good indicators of the position of a former ice front. In many places in Scotland, well-formed terminal moraines mark the maximum extent of the glaciers of the last really cold episode, the Loch Lomond Stadial.
A series of large terminal moraines at the west end of Loch Laggan near the Laggan Dam marks the final position of a powerful glacier that moved out of the Loch Treig glen and merged with the glacier in Glen Spean. The moraines here are formed chiefly of boulders and gravel. West of Spean Bridge, low moraine ridges occur near Brackletter and Leanachan. These are believed to mark the position of the retreating Spean glacier and to have formed beneath the lake waters.