The granite walls of Yosemite Valley bear the unmistakable signature of a massive ice sheet that once filled the space, carving a deep U-shaped trough where a river had previously cut a narrow V-shape. This transformation did not happen overnight but over thousands of years as the weight of accumulating snow and ice crushed, abraded, and scoured the bedrock beneath. As glaciers expanded during the Quaternary glaciations, they acted as powerful geological engines, dragging embedded rocks and sediments across the Earth's surface to create striations that still point the direction of ancient movement. These grooves and indentations serve as a historical record, allowing geologists to trace the path of ice that once covered vast regions like Fennoscandia and the southern Andes. While the Sahara today displays rare and very old fossil glacial landforms, the majority of the world's current glacial features were forged during the most recent ice ages, leaving behind a landscape that tells a story of immense power and slow, relentless change.
Mountains Shaped by Ice
High in the mountains, the interaction between multiple glaciers creates some of the most dramatic and jagged features on Earth, such as the sharp peaks known as glacial horns. These peaks form when several cirques, which are bowl-shaped indentations at the starting location of mountain glaciers, erode backward into a mountain until only a spiky high land remains between them. If the glacial action erodes through the narrow ridge connecting two cirques, a spillway or col forms, creating a pass that hikers might traverse today. The process also creates arêtes, which are spiky high land ridges between two glaciers, and hanging valleys that often result in waterfalls when glacial meltwater erodes the land partially. A classic example of this is the valley step, an abrupt change in the longitudinal slope of a glacial valley, which can be seen in the rugged terrain of the Pirin mountains in Bulgaria. These erosional features, including roches moutonnées and nunataks, demonstrate how the movement of ice can reshape the very skeleton of a mountain range.The Debris of Retreat
When the glaciers finally retreated, they left behind a chaotic freight of crushed rock and sand known as glacial drift, which formed characteristic depositional landforms across the landscape. This material, called glacial till, is composed of unsorted sediments ranging from fine dust to massive boulders that were eroded, carried, and deposited by the glacier some distance away from their original rock source. In New England, the stone walls of farms are built from glacial erratics, rocks that were dragged by a glacier many miles from their bedrock origin, serving as a testament to the ice's ability to transport heavy loads. As the ice melted, it built up mounds of till along the glacier, creating features such as terminal moraines that show how far the glacier extended, and lateral moraines that formed along the sides. The merger of lateral moraines from contributory glaciers created medial moraines, while other deposits formed eskers, which are built-up beds of subglacial streams that appear as small, string-like mounds left behind as the glacier retreats.