Glacier

ten thousand years under your fingertip.

advance the ice, carve the valley, retreat and read the evidence

Advancing the ice erodes the valley into a U-shape it would never wear on its own, drops a boulder far from where it started (an erratic), and retreating leaves a ridge of debris (a moraine) marking how far the ice once reached. Valleys, stray boulders, and ridges like these are still readable in real landscapes long after the ice that shaped them is gone.

Concepts: geology, erosion.

An interactive you play with one thumb, on Wundr.