Why are there seashells in the mountains?

That time, that place

Shen Kuo of the Northern Song served as a provincial official, a diplomat, and a director of waterworks. Pushed out of politics, he retired to a garden called Dream Brook and wrote down what he had seen and heard across his life — the Dream Pool Essays.

Why this question

Passing the cliffs of the Taihang mountains, he saw something strange: bands of seashells and rounded, egg-like pebbles running between the rock layers.

This was mountain country, hundreds of li from any sea. Shellfish do not walk uphill.

What was found

His conclusion: this had once been a shore. Silt and sand carried by rivers had piled up over ages into land, and the shape of the ground had changed that way.

The explanation assumes an enormous span of time — long enough that a human lifetime would show no change at all. Hutton would reach the same conclusion in Europe seven hundred years later.

He reasoned the same way about petrified bamboo found near Yan'an: since bamboo does not grow there now, the climate must once have been different. He was also the first to record that a compass needle does not point at true south but slightly off it.