In geology, a boulder is a rock with grain size of usually less than 10 inches in diameter. While 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 comes from Middle English “bulder” which was probably of Scandinavian origin such as dialectal Swedish “bullersten” meaning “noisy stone” (Imagine a large stone in a stream, causing water to roar around it) from “bullra” (to roar, cf. Dutch “bulderen”, with the same meaning) and “sten” (stone).
