Why Do Hillsides Slide?

Step 1 of 5

Gravity pulls along the slope, not just down.

On a slope, part of the pull acts sideways, down the hill. The steeper the slope, the bigger that sideways share, and the harder it is to hold.

Step 2 of 5

Friction and roots hold it back.

Grains catch on each other, and plant roots stitch the whole surface together. As long as that holds more than gravity pulls, nothing moves at all.

Step 3 of 5

Water is what usually tips it.

Water fills the gaps between grains and holds them apart, so they stop gripping each other. Heavy rain adds weight and removes grip at the same time.

Step 4 of 5

Cutting the bottom out does it too.

A river or a road that cuts into the foot of a slope removes what was holding the whole thing up. The slope above then has nothing to rest on.

Step 5 of 5

The warnings are usually there.

Slopes creep before they slide: fences lean, walls crack, trees curve at the base. Ground that has slipped once is far more likely to slip again.

Curved tree trunks on a hillside are a giveaway. The ground crept while the tree kept growing upward, so the trunk records the movement.