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:''This article is about the large rocks. For other uses, please see boulder (disambiguation).''


In geology, a 'boulder' is a rock with grain size of usually no less than 256 mm (10 inches) diameter. While a boulder may be small enough to move or roll manually, others are extremely massive.
In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called rocks or stones. The word boulder comes from Middle English "''bulder''" which was probably of Scandinavian origin such as dialectal Swedish "''bullersten''" meaning "''noisy stone''" (Imagine a large stone in a stream, causing water to roar around it) from "''bullra''" (to roar, cf. Dutch "''bulderen''", with the same meaning) and "''steen''" (stone).
In places covered by ice sheets during Ice Ages, such as Scandinavia and Northern North America, ice has moved and formed granite boulders (glacial erratics). One of the largest is used as the pedestal of the Bronze Horseman in Saint Petersburg.
Some famous rock formations involve giant boulders exposed by erosion, such as the Devil's Marbles in Australia's Northern Territory, the Wairere Boulders in New Zealand, where boulders only form an entire valley, and The Baths on the island of Virgin Gorda in the British Virgin Islands.
Bouldering is a sport that involves climbing on boulders.