Rock fragments, particles, or grains larger than 200 mm in size (British soil scale) or greater than 256 mm in size (North American scale). Boulders are the largest grain size within the category of gravel. The term likely derives from the Swedish term “bullersten” or Middle English term “bulderston.”
Boulders range in size from a minimum of 200 or 256 mm to isolated pieces weighing multiple tons (megaboulders or megaclasts). Glacial deposits around the northern hemisphere commonly host boulder sized clasts in the form of isolated erratics or within chaotic multitextural deposits such as ice-contact gravel features. Large glacial erratics are static features generally avoided during construction efforts and more often a tourist attraction (e.g., Okotoks erratic in Alberta, Canada). Typical natural processes capable of transporting boulders include landslides (e.g., debris flows, rock avalanches), lahars, tsunamis, flash floods (including natural and artificial dam...