A river cuts a valley over millions of years into a narrow, V-shaped channel, following the path of fastest-flowing water. A glacier, moving through the exact same kind of terrain, carves something structurally different: a wide, U-shaped trough with steep, nearly vertical walls and a flat floor — different tools, doing fundamentally different work.
Water cuts down. Ice grinds outward.
A river's erosive power concentrates in its fastest-moving channel, cutting downward and creating a narrow V-shaped cross-section over time, with the valley's shape closely tracking the path of the stream itself. A glacier moves as a mass, filling a valley from wall to wall, and erodes through two distinct mechanisms: abrasion, where rock fragments embedded in the ice's base scrape and polish the underlying bedrock like sandpaper, and plucking, where the glacier freezes onto cracked bedrock and physically tears pieces away as it moves. Because the glacier's whole mass is in contact with the valley's sides as well as its floor, it widens and deepens the valley more evenly across its full width, producing the flat-bottomed, steep-sided U-shape.
How to read a landscape's glacial history long after the ice is gone
Geologists can identify a valley as glacially carved thousands of years after the ice retreated, purely from its shape and a handful of other tell-tale features: hanging valleys, where a smaller tributary valley — carved by a smaller glacier with less erosive power — sits abruptly above the main valley floor, sometimes ending in a waterfall; cirques, bowl-shaped depressions where a glacier originally formed at the head of a valley; and glacial till, unsorted mixtures of rock and sediment a glacier deposits as it melts, distinct from the sorted, layered sediment a river leaves behind.
What we're still unsure about
Reconstructing exactly how fast a given glacier moved, and how much of a landscape's final shape came from one long glacial period versus several separate advances and retreats over hundreds of thousands of years, is often difficult to pin down precisely from the landscape evidence alone. Different glacial episodes can overprint and partially erase evidence of earlier ones, and glaciologists still rely on a combination of dating methods, some more reliable than others depending on the material available, to reconstruct a full glacial history for a given landscape — an area of ongoing refinement rather than settled certainty for many sites.
This sits inside Glacial Processes & Landscapes, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.