In geology , a BOULDER is a rock fragment with size greater than 25.6 centimetres (10.1 in) in diameter. Smaller pieces are called cobbles and pebbles . 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_ is short for _boulder stone_, from Middle English _bulderston_ or Swedish _bullersten_.
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. 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 .
* ^ Wentworth C.K. (1922). "A scale of grade and class terms for clastic sediments". _Journal of Geology_. 30 (5): 377–392. JSTOR 30063207 . doi :10.1086/622910 . * ^ _A_ _B_ "Boulder". Dictionary.com. Retrieved 24 August 2013. * ^ boulder. (n.d.) Online Etymology Dictionary. Retrieved December 9, 2011, from Dictionary.com