Moraine

• 1. Introduction

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  Moraine is the name geologists give to the debris a glacier carries, shapes, and eventually leaves behind. Picture a slowly moving river of ice grinding through a mountain valley, scraping rock from the walls, hauling boulders the size of cars, and depositing everything in ridges and sheets that can survive for thousands of years after the ice itself has gone. The word comes from the Savoyard Italian morena, meaning a mound of earth, and from there back to the Provençal morre, meaning snout. That root is fitting: moraines are, in a sense, the footprints of a glacier's snout. What forces actually build these landforms? How many distinct varieties exist, and what does each one tell us about the ice that made it? The answers stretch from the Alps to the Arctic, from northern Sweden to the Yukon.

• 2. A Word From Savoyard Mountains

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  Horace Bénédict de Saussure introduced the term moraine into formal geology in 1779, borrowing a word already in use among the people who lived alongside Alpine glaciers. The chain of borrowing is itself a small map of cultural geography. French picked up moraine from Savoyard Italian morena. Savoyard Italian drew it from the Provençal morre, denoting a snout or projecting face. And Provençal traced the shape back further to Vulgar Latin murrum, meaning a rounded object. Every step of that linguistic journey points at something physical: a protruding mass of debris pushed to the front or side of moving ice. De Saussure's decision to anchor the term in geology gave scientists across Europe a shared vocabulary for describing features they had been struggling to name. That shared vocabulary would eventually underpin the entire field of glacial geomorphology.

• 3. What Moraines Are Made Of

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  Glacial till is the raw material of most moraines, and till defies easy description. Unlike river sediment, which water sorts by size as it travels, till is unstratified and unsorted. A single deposit might hold particles ranging from silt-fine glacial flour all the way up to boulders. The individual fragments tend to be sub-angular to rounded in shape, worn by the slow violence of ice movement rather than the quicker abrasion of flowing water. When boulders dominate, geologists sometimes call the deposit boulder clay. The fine-grained matrix binding everything together is glacial flour, ground to powder by the weight and motion of the ice above. Moraines can appear on the surface of a still-active glacier or as piles and sheets of debris left wherever the ice has melted back. Either way, the unsorted character of the material is the signature that tells a geologist ice, not water or wind, was the transport agent.

• 4. How Ice Builds Landforms

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  Moraine-forming processes divide loosely into passive and active. Passive processes place chaotic debris from the glacier's surface onto the landscape with little reworking, typically producing hummocky moraines that look like a rumpled blanket of hills. Active processes, grouped under the term glaciotectonism, involve the ice itself moving and deforming sediment directly. Push moraines and thrust-block moraines both result from this more forceful action; they are often built from till mixed with reworked material that was in front of the glacier before the ice arrived. A third pathway involves meltwater: streams flowing out from the ice margin can deposit fans of sand and gravel that coalesce into a long bank tracing the glacier's former edge. In permafrost regions, an advancing glacier can shove thick layers of frozen sediment into what is called an arctic push moraine. Reworking of any of these deposits by later processes can concentrate heavy minerals into placer deposits; in the southernmost reaches of Chile, moraines have served as a source of placer gold.

• 5. A Map of Moraine Types

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  Lateral moraines run as parallel ridges along a glacier's flanks. Debris reaches them either by frost shattering of the valley walls above or by subglacial material melting out and migrating to the sides. These ridges can rise as high as 140 metres above the valley floor, extend up to 3 kilometres in length, and steepen sharply near the ice margin, reaching slopes of up to 80 degrees. Further from the glacier, the same ridge typically settles to slopes between 29 and 36 degrees. Ground moraines form wherever till accumulates at the base of the ice, creating gently rolling plains and hills with relief generally less than 10 metres; they occupy the areas between the more conspicuous end moraines. Rogen moraines, sometimes called ribbed moraines, look like tiger stripes from the air because the depressions between their ribs fill with water. They are named after Lake Rogen in Härjedalen, Sweden, the type locality for the landform. Closely related, de Geer moraines are ridges up to 5 metres high and 10-50 metres wide. Gerard De Geer first described them in 1889, and the Kvarken archipelago has a particularly high density of them. Washboard moraines, also called minor or corrugated moraines, sit low on the landscape, their ridges spaced roughly 100 metres apart and resembling, from altitude, the surface of an old laundry board. Veiki moraines build irregular landscapes of ponds and plateaus ringed by banks; they are common in northern Sweden and parts of Canada and form when ice covered by a thick debris layer melts unevenly.

• 6. Terminals, Medials, and What They Record

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  Terminal moraines mark the farthest point a glacier ever reached. Glaciers function much like a conveyor belt, carrying debris from the upper ice surface down to the snout and depositing it there; the longer the snout holds its position, the more material piles up. When the glacier pauses partway through a retreat rather than staying at its maximum extent, it produces a recessional moraine, a smaller ridge that cross-cuts a valley behind the terminal. In sequence, recessional moraines read like a diary of the glacier's pauses on its way back upslope. A medial moraine forms by a different mechanism entirely: when two glaciers converge, the debris riding the inner edges of both ice streams merges into a single band that travels down the center of the combined glacier. The Kaskawulsh Glacier in Kluane National Park, Yukon, carries a medial moraine ridge a full kilometre wide. Supraglacial moraines sit on top of actively moving ice, fed by debris falling from valley walls and by material rising through the ice in the ablation zone where the glacier is losing mass faster than it gains it. When a terminal moraine is left exposed after the ice retreats for good, postglacial erosion can dismantle it entirely, erasing the record it once held.