A Boulder is a large rock, greater than 256mm (10") in diameter. While a boulder may be small enough to move or roll manually, others are extremely massive. Some famous rock formations involve giant boulders exposed by erosion, for example the Devil's Marbles.
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A jumble of boulders in an Australian creekbed
Some people choose to climb larger-sized boulders for recreational purposes or as a method of training for mountaineering and free climbs. This practice is known as bouldering.
Example of an extremely large boulder - note size of tree for comparison.
There are several towns in America named Boulder or with boulder in their names. See