How did large boulders end up on hillsides far from similar rocks?

Most are glacial erratics—rocks carried by ice sheets during the last ice age and dropped when the ice melted around 11,700 years ago. Glaciers plucked blocks from bedrock and transported them like a slow conveyor belt, sometimes moving them tens or even hundreds of kilometres before abandoning them.

How do you tell if a boulder is a glacial erratic or just local rock?

Look for a mismatch between the boulder and the surrounding bedrock—like granite sitting on limestone. Not every hillside boulder is an erratic though; some are just weathered remnants of local bedrock or rockfall from nearby cliffs.

Why do some boulders balance on narrow pedestals of different rock?

These perched blocks form when the harder erratic protects the rock directly beneath it while softer surrounding rock erodes away over thousands of years. The Norber erratics in Yorkshire are classic examples, where greywacke boulders sit on limestone pedestals that stand above the weathered landscape around them.

How the Hills Got Here

Where the Boulders on the Hills Came From

Glaciers picked up blocks of rock and carried them far from where they formed before leaving them behind as the ice melted. These glacial erratics reveal both the direction of ice flow and the distant rock they came from.

27 August 20266 min read