Boulders

A boulder is a rock fragment large enough to distinguish from smaller stones and to require mechanical means to move. In geomorphology a common threshold is about 256 millimeters (10 inches) in diameter, though definitions vary by field.

Boulders form by weathering of larger rock bodies and by fragmentation through processes such as freeze–thaw, exfoliation, or chemical weathering. They may also be transported from their original location by gravity, rivers, landslides, or glaciers. When rocks are transported by glaciers and deposited far from their source, they are called glacial erratics.

Boulders occur in many settings worldwide, including talus slopes beneath cliffs, moraines in glacial landscapes, riverbanks, deserts, and coastal regions. Freshly broken boulders tend to be angular; prolonged transport in water or ice can round their edges. Over time they weather and may shed smaller fragments, contributing to soil formation.

Ecologically, boulders create microhabitats for lichens, mosses, fungi, insects, and small vertebrates. They can influence moisture retention and vegetation patterns around them. In engineering and land management, boulders are used for landscaping, erosion control, and as natural barriers. They can pose rockfall hazards on unstable slopes and require stabilization in construction and road-cutting projects.

In human use, boulders serve as landscaping elements, foundation stones in historic structures, and raw material for sculpture. They also function in erosion control and as natural barriers in some landscapes.