Boulder

In geology, a boulder is a rock fragment with size greater than 256 millimetres (10.1 in) in diameter.[1] Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive.[2] In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called rocks (American English) or stones (In British English a rock is larger than a boulder). The word boulder is short for boulder stone, from Middle English bulderston or Swedish bullersten.[3]

This balancing boulder, "Balanced Rock" stands in Garden of the Gods park in Colorado Springs, Colorado, United States.

Boulder in British Columbia, Canada

Boulder in a bay in Bic National Park, Québec

Kämmenkivi stone on the Pisa hill in Kuopio, Finland

Devonian conglomerate in Scotland with clasts up to boulder in size

In places covered by ice sheets during Ice Ages, such as Scandinavia, northern North America, and Siberia, glacial erratics are common. Erratics are boulders picked up by ice sheets during their advance, and deposited when they melt.[2] They are called "erratic" because they typically are of a different rock type than the bedrock on which they are deposited. One of them is used as the pedestal of the Bronze Horseman in Saint Petersburg, Russia.

Some noted rock formations involve giant boulders exposed by erosion, such as the Devil's Marbles in Australia's Northern Territory, the Horeke basalts in New Zealand, where an entire valley contains only boulders, and The Baths on the island of Virgin Gorda in the British Virgin Islands.

Boulder-sized clasts are found in some sedimentary rocks, such as coarse conglomerate and boulder clay.

The climbing of large boulders is called bouldering.