Monster Manual, p. 139
False Appearance. While the galeb duhr remains motionless, it is indistinguishable from a normal boulder. Rolling Charge. A galeb duhr gains a 6 squares speed when rolling, which increases to 12 squares if rolling downhill. If the galeb duhr rolls at least 4 squares straight toward a target and then hits it with a slam attack on the same turn, the target takes an extra 7 ( 2d6 ) bludgeoning damage. If the target is a creature, it must succeed on a DC 16 Strength saving throw or be knocked prone.
Slam. Melee Weapon Attack: 1d20+8 to hit, reach 1 sq., one target. Hit: 12 ( 2d6+5 ) bludgeoning damage. Animated Boulder - Galeb Duhr (1/Day). The galeb duhr magically animates up to two boulders it can see within 12 squares of it. A boulder has statistics like those of a galeb duhr, except it has Intelligence 1 and Charisma 1, it can't be charmed or frightened, and it lacks this action option. A boulder remains animated as long as the galeb duhr maintains concentration, up to 1 minute (as if concentrating on a spell).