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how do glaciers carve out valleys
Picture Yosemite, with cliffs nearly a kilometre tall. A river began that valley, but slow ice reshaped it, leaving a wide U instead of a river's V.
Don't picture a bulldozer. Ice is soft. A glacier is a river of ice, creeping downhill under its own weight, a few metres to hundreds of metres a year.
First, a river. It cuts straight down along its narrow bed, so its valley is a V, steep sides sloping to a point.
A glacier is different. Ice fills the valley wall to wall, so it wears the sides as well as the floor, and the V is hollowed out into a U.
But how does soft ice cut rock? With tools. Stones from the ground freeze into the glacier's base, and the flowing ice drags them like sandpaper.
That grinding is abrasion. It scratches grooves into the rock and crushes it to a fine grey powder called rock flour, which turns glacier streams milky.
The second tool is plucking. Meltwater seeps into cracks and freezes, the ice grips a whole block, and as the glacier moves it tears the block free.
So a glacier is a slow tool belt: ice flows, stones grind, blocks tear free, and over thousands of years a river's V becomes a glacier's U.
Ice is soft, but a river of ice dragging stones is the slowest, strongest chisel on Earth.