How did glaciers carve valleys like Yosemite?

Glaciers carved valleys like Yosemite by filling them with hundreds of metres of moving ice that ground the rock away, turning a river's narrow V into a broad U with steep, straight walls and a flat floor. A glacier is ice thick enough to flow, slowly, under its own weight. As it creeps downhill it does two things. It plucks: meltwater seeps into cracks in the bedrock, freezes and thaws, and loosens blocks that the ice then carries off, some up to three metres across. And it abrades: the ice, armed with all that rock, works like sandpaper on the valley floor and walls.

It isn't quick. Turning a V-shaped valley into a U-shaped one takes between 10,000 and 100,000 years. And the deeper the ice, the harder it cuts, because thicker ice flows faster and erosion is directly proportional to its thickness. In the Yosemite area, glaciers reached depths of up to 1,200 metres. When the ice melts, small side valleys whose glaciers carved less are left hanging high on the walls, often pouring out as waterfalls. Where a U-shaped valley reaches the sea, it becomes a fjord.

This was once fiercely disputed. Josiah Whitney, the chief of the California Geological Survey, insisted that Yosemite Valley formed when its floor sank. John Muir, who was exploring the area at the same time, argued for glaciers. Whitney derided him as an 'ignoramus' and a 'mere sheepherder', but most scientists eventually sided with Muir.