In geology, a boulder is a rock fragment with size greater than 256 millimetres 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.

What does the word boulder means?

: a very large stone or rounded piece of rock. See the full definition for boulder in the English Language Learners Dictionary. boulder. noun. boul·​der | \ ˈbōl-dər \

What is a boulder rock?

A boulder is defined as any rock larger than 16” in diameter. They are available in two basic shapes: round and angular. Round boulders have smooth edges and curves. They are water-washed or river-run surface stones of granite and sandstone, worn over the eons by wind, sand and rain.