Boulder

A boulder is a large, rounded rock fragment broken from bedrock or a mountain and found elsewhere. The term is used for rock fragments too large to be held in the hand, generally larger than the smaller stones and gravel found mixed with soil and sediment.

In geology, boulders are defined as part of the size classification system for fragmented rock material. Size classification is based on diameter; by standard scientific measurement, boulders are fragments with diameters greater than about one quarter metre. Smaller categories below include cobbles, pebbles, gravel, and sand.

Formation

Boulders are typically produced by the breakdown of bedrock. Multiple processes contribute: chemical weathering from water, freeze-thaw cycles from ice and water expansion, thermal weathering from heating and cooling that fractures rock, and mechanical action from tree roots. As fragments break off, gravity carries the rock downslope, and accumulations of boulders can be found at the bases of hills.

Transport and distribution

Large boulders cannot be easily moved, but powerful streams, avalanches, and glaciers can carry them considerable distances. Glaciers transport boulders to areas of different rock type; these are called erratics and provide evidence of past glacial movement. Boulders carried within flowing glaciers have their surfaces rounded and polished in systematic patterns, and the wear patterns indicate the distance travelled.

Significance and uses

Boulders affect water flow in streams and create aquatic habitats. On hillsides, boulders present hazards to roads and settlements; engineers therefore build barriers and deflection systems. Culturally, certain boulders have become landmarks, and rock bouldering (climbing on natural boulder faces) is a recreational activity.