Ice in Motion: How Glaciers Shape Our World

Look at a photograph of the Lake District in northern England or Yosemite Valley in California. Both have something strange: the valleys are wide, flat-bottomed, and steep-sided — almost like someone scooped them out with a giant spoon. Rivers couldn't do this. Rivers carve narrow, V-shaped valleys. So what made these enormous, U-shaped troughs? Think about it before reading on.

A glacier is a large, slow-moving mass of ice that forms on land when more snow falls each winter than melts each summer. Over hundreds of years, layers of snow compact under their own weight, expelling air and turning into dense glacial ice. Eventually the ice is thick and heavy enough to flow downhill under gravity — a process called glacial movement. Glaciers move in two ways. First, the ice at the base melts slightly under pressure, creating a thin film of meltwater that lets the glacier slide — this is called basal sliding. Second, the ice crystals inside the glacier slowly deform and shift past each other — this is called internal deformation. Together, these processes move a glacier anywhere from a few centimetres to several metres per day. As a glacier moves, it erodes — wears away — the rock beneath and beside it through two powerful processes. Plucking occurs when meltwater seeps into cracks in bedrock, refreezes, and bonds to the glacier. As the ice moves forward, it rips chunks of rock away. Abrasion happens when those rock fragments, frozen into the base of the glacier, grind the bedrock below like sandpaper, smoothing and scratching it. Scratches left in bedrock by abrasion are called striations, and geographers use them as evidence that glaciers once existed in a place. Erosion creates distinctive landforms. A corrie (also called a cirque) is a bowl-shaped hollow high on a mountainside where a glacier begins — snow and ice collect there and erosion deepens the bowl. When two corries erode back-to-back on a mountain ridge, they sharpen it into a knife-edged arête. If three or more corries erode towards the same peak, they sculpt a sharp, pyramidal horn — the Matterhorn on the Swiss-Italian border is a perfect example. As the glacier flows downhill, it widens and deepens existing river valleys into the U-shaped glacial troughs you noticed in the starter, sometimes leaving smaller tributary valleys stranded high on the walls — these are called hanging valleys. Glaciers also deposit — drop — the material they carry. The mixture of unsorted rocks, sand, and clay deposited directly by ice is called till. Ridges of till left at the snout (front end) of a glacier are terminal moraines, which mark the furthest point the glacier reached. Long, smooth, teardrop-shaped hills of till — called drumlins — are deposited in swarms and their pointed ends show the direction the ice travelled. Today, glaciers exist in places like Antarctica, Greenland, Alaska, and the high Alps. But during the last ice age, which ended roughly 11,700 years ago, ice sheets covered much of northern Europe, Canada, and North America. The landscapes of Scotland, Wales, Scandinavia, and the Great Lakes region of North America were all shaped by this ancient ice.