I think you'd set the difficulty for the bridge at the same strength as the zone border (which has to be a hole of some sort). What you are doing is essentially making are horizontal "zone border" that goes over the existing one. If the pit is an entire zone in itself, then it has to be a zone-wide bridge.
So a long chasm might be a +3 (good) border, so you'd need 3 shifts to make a bridge across it.
Fire can be used to make ice bridges over water or to stifle a fire border so there's a passage. Water can bore a hole through a wall or the like (but the difficult of this might be higher than the actual border, since you have to break whatever the wall is made of). Earth or Spirit can go over pits, chasms, and the like.
This all seems pretty quick and dirty to me, and I think it is ok for Evocation, imho.