How Does a River Cut a Gorge?

Step 1 of 5

Water is not what does the cutting.

Plain water barely wears rock at all. What cuts is the sand and gravel a river drags along its bed, scraping across the bedrock like grit on sandpaper.

Step 2 of 5

Depth wins when the land is rising.

A gorge forms where the river cuts down faster than the walls can crumble inward. That happens when the land is being lifted, giving the river more to cut through.

Step 3 of 5

Hard rock keeps the walls standing.

Soft rock slumps into a gentle slope. Hard rock stands as a cliff, so the deepest gorges are cut where the river is strong and the rock is stubborn.

Step 4 of 5

Waterfalls walk backwards.

Where a river crosses from hard rock to soft, the soft rock is scooped out from under the fall and the lip collapses. The whole waterfall retreats upstream, year after year.

Step 5 of 5

The walls are a stack of dates.

Cutting downward exposes older and older rock. The bottom of a deep gorge can be hundreds of millions of years older than the rim you are standing on.

Some gorges were cut catastrophically fast, when an ice-dammed lake burst and emptied across the landscape in a matter of days.