Monster Manual, page 139
Speed increased to 30 ft when rolling, 60 ft when rolling downhill False Appearance. While the galeb duhr remains motionless, it is indistinguishable from a normal boulder. Rolling Charge. If the galeb duhr rolls at least 20 feet 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: +8 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. Animate Boulders (1/Day). The galeb duhr magically animates up to two boulders it can see within 60 feet 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).
Hill, Mountain