New England's stone walls are full of boulders carried for miles by ice
Many rocks stacked into New England's old field walls are glacial erratics, stones dragged far from their home bedrock by ice sheets. Glaciers act as both sculptors and delivery trucks, carving bowls and sharp peaks as they advance and dumping hills, ridges and lakes as they melt back.
Most glacial landforms visible today were made by the great ice sheets of the Quaternary glaciations. Scandinavia and the southern Andes are covered in them, while the Sahara preserves rare, very ancient examples. As ice thickens and flows, its weight grinds and scours the ground beneath. Small stones frozen into the base scratch grooves called striations, which point in the direction the ice travelled, much like arrows left behind on the bedrock.
Mountain glaciers leave a distinctive set of shapes. Each one starts in a cirque, and when it melts it leaves a bowl in the mountainside, sometimes holding a small lake called a tarn. Where two glaciers grind against the same ridge they sharpen it into an arête, and several arêtes meeting form a horn. Glaciers turn river valleys into broad U shapes; flood one with seawater and you have a fjord. Side valleys can be left stranded high above the main trough, often spilling waterfalls.
Retreat builds a different landscape from the rubble. Unsorted debris called till piles up as moraines along a glacier's snout and flanks, the terminal moraine marking its farthest reach. Streams flowing under the ice leave winding ridges known as eskers, and sediment falling into holes in the ice forms mounds called kames. Stranded blocks of ice melt into kettle lakes, and debris can dam streams: Jenny Lake in Grand Teton National Park is held back by a moraine, as is Jackson Lake, though a man-made dam enlarges it.
Not every landform blamed on ice deserves the label. Erling Lindström argues that the smoothed rock humps called roches moutonnées were mostly shaped before glaciation, since similar forms occur in tropical East Africa and Australia. The glacial buzzsaw idea, that ice planed off Norway's high flat surfaces, struggles to explain why those surfaces form a series of steps. Even the Gulf of Bothnia and Hudson Bay, sitting where ice sheets once centred, owe more to tectonics than to glacial erosion.
Source: Glacial landform
