thumb|This [[Balancing rock|balancing boulder, "Balanced Rock", stands in Garden of the Gods park in Colorado Springs, Colorado, United States.]]
thumb|Boulder in [[British Columbia, Canada]]
thumb|Kämmenkivi stone on the Pisa hill in [[Kuopio, Finland]]
thumb|Balanced granite boulders at [[Hyderabad, India]]
In geology, a boulder is a rock fragment with size greater than 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.
Etymology
The word boulder derives from boulder stone, from Middle English bulderston or Swedish bullersten. It was previously sometimes spelled bowlder, which survives as a less common variant spelling.
About
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.