A jumble of boulders in an Australian creekbed

In geology, a boulder is a large rock, greater than 256 mm (10 in) in diameter (see grain size for standards in use). 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 in Australia's Northern Territory.

Example of an extremely large boulder—note size of tree for comparison

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.

Towns

There several towns in America named Boulder or with boulder in their names. See

• Boulder, Colorado
• Boulder, Utah
• Boulder City, Nevada.

• Boulder, Western Australia