Glaciers Shape the Land

Look at this valley from above. Wide, flat floor. Steep walls on both sides — going straight up like the inside of a cereal bowl. Here's the strange thing: there's a little river running through the bottom of it. But that river is way too small to have carved a valley this huge. So if the river didn't do it… what did? The answer is something bigger, heavier, and a lot slower than water — and at one time, half of California's Yosemite was buried under it.

A glacier is basically a river — except it's made of ice. Here's how one is born. Up in the mountains, snow falls every year and never fully melts. Layer on layer on layer. Eventually, the snow at the bottom gets crushed by all the snow on top of it, until it's squeezed so hard it turns into solid blue ice. And once that ice is heavy enough, it starts to flow downhill — really slowly. Like cold toothpaste. So slowly you can't see it move. But it does — and it doesn't stop.

Now here's where it gets cool. A river cuts a valley shaped like the letter V — pointy at the bottom, slopey sides. That's because water only touches the very bottom of the channel as it flows. But a glacier is huge and fat. It fills the entire valley from wall to wall. So it grinds the floor AND the walls at the same time. And when it's done, the valley is shaped like the letter U — flat floor, steep sides. Yosemite Valley? That's a U. Glacier carved.

While a glacier is sliding downhill, it scoops up everything in its path — sand, pebbles, big boulders. Some get pushed in front of it. Some get carried inside the ice itself, like raisins frozen in a popsicle. And when the glacier finally melts away, all that stuff has to go somewhere. It drops, right there, in a pile called a MORAINE. And the huge boulders that landed in totally random spots? Those are called ERRATICS — orphan rocks that travelled hundreds of kilometres on the back of an ice river.

Now look at the underside of a glacier. It's not smooth — it's full of rocks frozen right into the ice. Some are pebbles. Some are the size of a school bus. So when the glacier slides over the bedrock underneath, those embedded rocks drag along like teeth on a saw. They carve long, parallel scratches called STRIATIONS into the rock. Centuries later, you can find those scratches and tell exactly which direction the glacier was flowing — like reading footprints in dried mud.

Your turn. You have a V-shaped river valley with mountains on both sides. Hit SEND GLACIER and watch a slow river of ice flow through, grinding the floor flat. Then flip the switch to INTERGLACIAL — the ice melts away, and you'll see what it left behind: a U-shaped valley, a pile of dropped rocks, and a few erratic boulders just sitting there. Try it. That's how every glacial valley you'll ever see got made.

So here's the recap. A glacier is a slow river of ice, heavy as a mountain, with rocks frozen into its base like sandpaper. Give it tens of thousands of years and three things happen: it grinds the valley below it into a U shape, it dumps the rocks it picked up into a pile called a moraine, and it scratches long parallel lines into the bedrock as it slides. So when you see a U-valley, a random giant boulder in a field, or a striped slab of rock — you're reading a glacier's fingerprints. And about half of Yosemite was made exactly this way.