You gain a +2 enhancement bonus to your CMD to resist bull rush, drag, reposition, trip, and overrun attempts as long as you are touching the ground. This bonus increases by +1 for every five wizard levels you possess. In addition, you gain a +1 insight bonus on melee attack and damage rolls whenever both you and your foe are touching the ground. At 20th level, earth and stone do not block the line of effect of your spells, although they do still block your line of sight.
