Nelson-Denny Practice Test 1

Rivers of Ice

¹A glacier is a large mass of ice that forms on land where, year after year, more snow falls in winter than melts in summer. As fresh layers accumulate, the weight of the snow above presses the older snow below into a granular material called firn, and over decades the firn is squeezed into dense glacial ice. Although ice seems rigid, a glacier is never truly still. Once the ice becomes thick enough, roughly fifty meters or more, it begins to move downhill or outward under its own weight.

²Glaciers move in two main ways. In the first, called internal flow or plastic deformation, the ice crystals deep within the glacier slowly slip and reshape under pressure, so that the ice behaves somewhat like a very stiff liquid. The upper layer, about fifty meters thick, is too lightly loaded to flow in this way. It remains brittle and is carried along on top of the moving ice beneath, often cracking into deep crevasses where the glacier passes over uneven ground. In the second kind of motion, called basal slip, the entire glacier slides over its bed. This sliding is aided by meltwater at the base, which reduces friction between ice and rock. Because meltwater is more plentiful in warm seasons, many glaciers move faster in summer than in winter.

³As it moves, a glacier acts as a powerful agent of erosion. Two processes are mainly responsible. In plucking, meltwater seeps into cracks in the bedrock beneath the glacier and refreezes, attaching loosened blocks of rock to the ice, which then carries them away. In abrasion, rock fragments frozen into the bottom of the glacier scrape the bedrock like the grit on sandpaper, leaving long parallel scratches called striations and grinding some of the rock into a fine powder known as rock flour. Geologists use the direction of striations to reconstruct the paths of glaciers that melted thousands of years ago.

⁴The landforms carved by glaciers are distinctive. A river typically cuts a narrow valley with a V-shaped cross section, wearing mainly downward into its bed. A valley glacier, by contrast, fills the whole valley and erodes its floor and sides at once, widening and deepening it into a broad U shape. Smaller tributary glaciers do not cut down as deeply as the main glacier they join, so when the ice melts, their valleys are left perched high above the main valley floor as hanging valleys, from which waterfalls often spill. At the head of a valley glacier, plucking can hollow out a bowl-shaped basin called a cirque.

⁵Glaciers also build landforms from the material they carry. Because ice, unlike running water, cannot sort sediment by size, the deposits that glaciers leave, called till, are a jumble of clay, sand, gravel and boulders mixed together. Ridges of till that pile up along the sides and front of a glacier are called moraines; an end moraine marks the farthest point the ice reached before it began to retreat. Large boulders dropped far from the bedrock they came from, known as erratics, gave early naturalists some of the first evidence that great ice sheets once covered regions that are free of ice today.