What Is a Glacier Moraine? A Pile of Everything the Ice Carried

Moraines are classified by their position relative to the glacier that made them, and each records something different:

• •Terminal or end moraine, a ridge across the valley marking the furthest advance of the ice, which is the single most useful feature for reconstructing past glaciation
• •Recessional moraines, a series of ridges behind the terminal one, each marking a pause during retreat, so their spacing records how the retreat proceeded
• •Lateral moraines, ridges running along the valley sides built from debris falling onto the ice edge, which remain as high benches long after the ice has gone
• •Medial moraines, dark stripes running down the middle of a glacier where two ice streams merge and their inner lateral moraines combine
• •Ground moraine, an uneven sheet of material dropped beneath the ice, which produces the hummocky landscape of formerly glaciated lowlands
• •Push moraines, where an advancing front bulldozes material ahead of itself and deforms it

Glacial deposits are recognisable because ice does not sort. Water sorts sediment by size, since a current carries fine particles further than coarse ones, which is why river and beach deposits are graded and layered. Ice carries everything at the same speed regardless of size, so it drops a chaotic mixture of clay, sand, gravel, cobbles and boulders together with no bedding at all, a material called till. Individual stones carry further evidence: they are frequently angular rather than rounded, since they were not tumbled in water, and many bear striations, straight scratches cut as the stone was dragged across bedrock under enormous pressure. Erratics, boulders of rock type entirely foreign to the area where they sit, prove transport over long distances and were among the first evidence that ice sheets had once covered regions now temperate, since no other mechanism could plausibly move a house-sized granite block onto limestone hundreds of kilometres away.

The recognition that large parts of Europe and North America had been under ice was a nineteenth-century argument and moraines were central to it. Erratics, till and polished striated bedrock were long explained by the biblical flood, with boulders supposedly rafted by icebergs, an interpretation that gave the deposits the name drift. Swiss naturalists working in the Alps could see living glaciers producing exactly these features and argued that the same processes had operated far beyond the current ice. Louis Agassiz presented the case forcefully from 1837, arguing for a former great ice age, and the theory was resisted before being accepted as the evidence accumulated, particularly the mapping of moraine sequences far from any mountains. Moraines are now used quantitatively: dating boulders on a terminal moraine by measuring isotopes produced in them by cosmic rays since exposure gives the date the ice stood there, which has built detailed chronologies of advance and retreat across the last glaciation.

Moraines shape modern geography in ways that are easy to overlook. They dam valleys and hold back lakes, and many mountain lakes sit behind a terminal moraine, which is also a hazard, since a moraine-dammed lake filling with meltwater can fail catastrophically in what is called a glacial lake outburst flood, a growing risk in the Himalaya and Andes as glaciers retreat. They control drainage and soil across formerly glaciated lowlands, with till producing heavy poorly drained soils and the associated outwash producing sandy well-drained ones, a distinction that determined agricultural patterns and settlement across northern Europe and North America. Long Island and Cape Cod are substantially built of moraine. In mountain areas, lateral moraines left high on valley sides after ice thinning are unstable and are a major source of landslides and debris flows. And the material itself is economically useful, with glacial sand and gravel being a primary source of construction aggregate across the northern hemisphere.

A glacier carries rock without sorting it and dumps it where the ice stops, producing terminal moraines marking the furthest advance, recessional ridges marking pauses during retreat, and lateral and medial ridges built from debris on the ice surface. The material, till, mixes every grain size with no bedding, and striated stones and far-travelled erratics identify it. Moraine evidence established that ice sheets once covered temperate regions, and moraine-dammed lakes are a growing flood hazard.