Glacial landform

• 1. Introduction

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  Glacial landforms are the marks left by glaciers on the surface of the Earth. Most of them are not being made today. They were carved during the Quaternary glaciations, when vast ice sheets ground their way across whole continents. Some places carry these scars in abundance. Fennoscandia and the southern Andes are crowded with them. Other places hold only ghosts of an icier past. In the Sahara, you can still find rare and very old fossil glacial landforms, relics of a climate that has long since vanished. So how does something as slow and silent as ice cut a mountain into a spike, hollow out a valley, or strand a single boulder hundreds of miles from the rock it broke off? And why do some of these features turn out not to be glacial at all? The answers begin with what happens when ice gets heavy enough to bite.

• 2. The Cutting Edge

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  Accumulating weight of snow and ice is what gives a glacier its teeth. As glaciers expand, that weight lets them crush, abrade, and scour everything beneath them, from loose rocks to solid bedrock. The features left behind are called erosional landforms, and the family is large. It includes striations, cirques, glacial horns, arêtes, trim lines, U-shaped valleys, roches moutonnées, overdeepenings, and hanging valleys.

  Striations are the simplest record of all. They are grooves and indentations scratched into rock outcrops as small sediments trapped on the bottom of a glacier drag across the Earth's surface. Their direction is a compass needle frozen in stone. The way the striations point tells you the way the glacier was moving.

  A cirque is where a mountain glacier is born. It is the starting location for the ice, and once the small glacier melts away it leaves a bowl-shaped indentation in the mountainside. A sequence of these bowls strung together has its own name, a cirque stairway. When a stream later fills a cirque after overdeepening, the water that gathers there becomes a tarn.

  U-shaped valleys are the signature of mountain glaciers, broad and trough-like where a river valley would have been narrow and sharp. When the sea floods one of these troughs to create an inlet, the valley earns a different name. It becomes a fjord. The high ground between two glaciers gets sharpened too, into a spiky ridge called an arête, and where glacial action erodes all the way through an arête, a spillway or col forms. Push several arêtes together at a single point and you get a horn, a sharp peak built where multiple glacier intersections meet.

• 3. What The Ice Drops

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  Retreat is when a glacier becomes a builder rather than a blade. As the ice pulls back, it abandons its freight of crushed rock and sand, the material known as glacial drift, and the depositional landforms it leaves are unmistakable. Many are made of glacial till, a jumble of unsorted sediments. Some pieces are quite large and others small, all eroded, carried, and dropped by the glacier some distance from the rock they came from.

  Moraines are the mounds where this till piles up along the glacier. The names depend on where the pile sits. A terminal moraine forms at the end of a glacier and marks how far the ice once reached. A lateral moraine runs along the sides. A medial moraine appears when lateral moraines from contributory glaciers merge into one. The catalogue of moraine types runs further still, through Pulju, Rogen, Sevetti, terminal, and Veiki.

  Eskers tell a watery story. An esker is the built-up bed of a subglacial stream, surviving as a small, string-like mound after the glacier retreats. A kame is its irregular cousin, a lumpy heap of sediments that had been deposited earlier by falling into an opening in the glacial ice. Drumlins and ribbed moraines also count among the shapes left behind by retreating glaciers.

  Meltwater is a sculptor in its own right. Landforms shaped by sediment that water deposited or reshaped are called fluvioglacial landforms, and they differ from glacial till in a telling way. Because water did the carrying, the sediments are more size sorted than till ever is. An outwash fan is a clear case, a braided stream spilling from the front of a glacier onto a flatter, lower plain of sediment. The most portable evidence of all sits in plain sight. The stone walls of New England are full of glacial erratics, rocks dragged by ice many miles from the bedrock where they began.

• 4. Water In The Hollows

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  Kettle lakes are born from a buried death. When a retreating glacier leaves behind a chunk of ice, on the surface or underground, that ice later melts and the depression it occupied fills with water. Other lakes owe their existence to debris rather than ice. Moraine-dammed lakes occur where glacial debris dams a stream or snow runoff, and Grand Teton National Park holds two examples in Jackson Lake and Jenny Lake. Jackson Lake is partly a human collaboration, enhanced by a man-made dam.

  A glacial valley can hold lakes in a chain. Paternoster lakes are a series of pools formed when a stream is dammed by successive recessional moraines, left as a glacier advances or retreats. The term glacial lake itself has a precise meaning here. It describes a lake that formed between the front of a glacier and the last recessional moraine, water caught in the narrow gap between the ice and the rubble it had pushed ahead of it.

• 5. The Ice Itself

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  Polar regions show that the glacier is a landform too, not only the maker of one. Apart from everything ice leaves behind, glaciers themselves are striking features of the terrain. Valley glaciers are those whose flow is hemmed in and directed by the valley walls around them. Higher up, the upper section of glacial ice cracks open into crevasses. And where the ice pours over a steep drop, it produces an icefall, the frozen equivalent of a waterfall.

• 6. When Ice Gets The Blame

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  Roches moutonnées may not be glacial at all, according to a thesis advanced by Erling Lindström. He argues these forms may have had most of their shape before glaciation ever arrived. The jointing that contributes to their shape typically predates glaciation, and roche moutonnée-like forms turn up in tropical areas such as East Africa and Australia, where glaciers were not the cause. At Ivö Lake in Sweden, rock surfaces exposed by kaolin mining and then weathered came to resemble roche moutonnée on their own.

  The glacial buzzsaw effect has drawn similar doubt. This is the idea that elevated flat surfaces were leveled by glaciation, and various scholars have rejected it. In Norway, the elevated paleic surface was proposed as a product of the buzzsaw, but the proposal is hard to square with the evidence. The paleic surfaces form a series of steps at different levels. The glacial cirques that are supposed to do the leveling line up with none of those steps, and neither the modern equilibrium line altitude nor the Last Glacial Maximum equilibrium line altitude matches any level of the composite paleic surface. The elevated plains of West Greenland are likewise unrelated to any buzzsaw.

  Some of the largest features blamed on ice were shaped by deeper forces. The Gulf of Bothnia and Hudson Bay are two great depressions sitting at the centre of former ice sheets, which makes the glacial explanation tempting. Yet both are known to be more the result of tectonics than of any weak glacial erosion, a reminder that sitting under an ice sheet does not prove the ice did the digging.