Key takeaways
- A moraine is rock and debris (till) carried and deposited by a glacier.
- Main types: lateral (along the sides), medial (where two glaciers merge), and terminal/end (at the snout).
- Moraines form ridges of loose, often unstable rock — tiring and tricky to travel.
- They mark current and former glacier extents, and are common on approaches to glaciated peaks.
From French 'moraine', from a Savoyard dialect word for a mound of earth.
How moraines form
A glacier is a slow river of ice that plucks and carries rock as it flows. That debris — gravel, sand, and boulders collectively called till — gets deposited wherever the ice drops it: along the glacier’s flanks, down its center, and at its end. The resulting ridges and mounds of loose rock are moraines.
Types of moraine
- Lateral — ridges along the sides of the glacier.
- Medial — a debris stripe down the middle where two glaciers merge.
- Terminal (end) — deposited at the glacier’s farthest advance, marking its snout.
Approaching a glaciated peak, a team grinds across a lateral moraine — a steep, shifting ridge of loose rock — picking careful footing and watching for rockfall before finally stepping onto the ice for glacier travel.
Why they matter
For travelers, moraines are tiring, unstable terrain similar to scree and talus but glacier-deposited. For reading the landscape, old terminal moraines stranded far below today’s ice reveal how far glaciers once reached before retreating.
The bottom line
A moraine is the rocky debris a glacier carries and dumps — in ridges along its sides, down its middle, and at its end. For mountaineers they're a familiar, loose, ankle-testing feature of glacier approaches; for geologists they map where ice once flowed. Either way, recognizing moraine types is part of reading glaciated terrain.
Frequently asked questions
What is a moraine?
A moraine is a deposit of rock, gravel, and sediment — collectively called till — that a glacier has transported and left behind. As ice flows it carries debris along, dropping it at the glacier's sides, middle, and end, building ridges and mounds of loose material.
What are the types of moraine?
The main types are lateral moraines (ridges along the sides of a glacier), medial moraines (a stripe of debris down the middle where two glaciers merge and their inner lateral moraines combine), and terminal or end moraines (deposited at the glacier's farthest advance, marking its snout).
Why do mountaineers care about moraines?
Moraines are common, awkward terrain on the approach to glaciated peaks — ridges of loose, shifting rock that are tiring and slow to cross, with rockfall and unstable footing. They also reveal glacial history; the position of old terminal moraines shows how far glaciers once reached before retreating.
Sources
- Glaciers & glacial landforms — USGS
- Mountain travel — The Mountaineers