How is a canyon formed?

How it works

Water looks soft, but a river carries sand and stones that scrape the riverbed like sandpaper that never stops. Each year it wears away only a tiny amount, but over hundreds of thousands or millions of years, the river can cut a very deep canyon.

If the land is slowly rising at the same time, the river has to keep cutting down, so the canyon becomes even deeper. Canyons in dry places often have very steep walls, because there is little rain to wash the sides down and wear them into gentle slopes.

The stripes in a canyon wall are layers of mud, sand and shells that settled long ago and hardened into rock. Usually, the lower a layer is, the older it is. The wall is like a history book standing on its end, with each layer a page.

Where to see it in New Zealand

In New Zealand, canyons are often called gorges. In the North Island's Manawatū Gorge, scientists think the Manawatū River is older than the ranges around it: as the land slowly rose, the river kept cutting down and now runs right through them. The Grand Canyon in the United States, carved by the Colorado River, is famous for its striped walls.

Try it together

In a sandpit or on a mound of garden soil, pour a slow, steady trickle of water from a watering can onto one spot. Watch the water carve a channel. Does it get deeper the longer you pour? Try building layers of different-coloured damp sand first to see 'rock layers' appear.