Bridge of Wood
by LibraryOgreSo, a more low-level spell solution for this, with higher-level options possible
Bridge of Wood
Level 3
Duration: Special
Effect: A bridge of wood, 15' * 30'
This spell creates a bridge made out of wood, 15 feet wide and up to 30 feet long. The wood itself is about 2' thick, but the spell will create the necessary bracing to ensure that the bridge is strong and stable. If the gap needing bridging is longer than 30', multiple castings can be used to extend it, 30' at a time.
The bridge lasts indefinitely, but only until its lifetime load is exceeded. Over the course of its duration, the bridge can accept 100,000cn of weight (about 5 tons); if a wagon weighing one ton is driven across it, that wagon will be able to cross five times before the bridge collapses. On the final trip, the bridge will allow crossing in proportion of the weight remaining to the weight of what's crossing it; if the bridge can only manage half a ton, that one-ton wagon will find the bridge collapsing when it is half way across. When the bridge starts to collapse, there is only one round to reach the other side (or section); longer than that round, and whatever is on the bridge will fall.
If Bridge of Wood is cast with a staff from a dryad's tree, its lifetime load is 200,000cn. If the branch was freely given by the dryad and her tree, then the bridge is permanent, with a maximum load of 50,000cn at any one time.
Bridge of Stone would be a 4th level version, with maybe a 300,000cn lifetime load. Bridge of Iron would be 5th, and then Bridge of Force (your spell) would be 6th. Obviously, Bridge of Wood (and maybe stone) should also be available to druids.