Earth Tremor

You strike the ground and unleash a tremor of seismic force, hurling up earth, rock, and sand. You choose whether the earth tremor affects a 30-foot line, a 20-foot cone-shaped spread, or a 10-foot-radius spread centered on you. The space you occupy is not affected by earth tremor. The area you choose becomes dense rubble that costs 2 squares of movement to enter. Dense rubble adds 5 to the DC of Acrobatics checks and adds 2 to the DC of Stealth checks. Creatures on the ground in the area take (min(10,CASTERLEVEL))d4 points of bludgeoning damage or half damage on a successful save. Medium or smaller creatures that fail their saves are knocked prone. This spell can be cast only on a surface of earth, sand, or stone. It has no effect if you are in a wooden or metal structure or if you are not touching the ground.