Scorching Ray does an average of 42 damage. If you're near a Bard it can easily do 60 or more (67.5 with my Bard's standard buffs). I probably wouldn't allow Pit+Wall and Pit+Object combos to do anything exciting, but they certainly shouldn't do more damage than it would take to destroy the Wall or Object. Wall of Force needs to be vertical but seems like it could create a pretty deadly "log splitter" effect above the pit. I'd think that at most it would force a Large creature to squeeze as it comes out of the pit though. Huge creatures are too big to fall into the pit.
@Ravingdork - The ship's deck is the "floor" of that area. While it could be moved or manipulated somehow that's probably not expected. I'm not sure if a throw rug is part of the "floor". If your boots are on the floor do the soles disappear as the pit forms? I'd guess not because the boots are attended. Is a rug you're standing on attended? How about if it is a flying carpet? Fairness is probably a better goal than consistency here.
@Remy - The manhole situation is interesting. I wonder if the cover wouldn't fall into the Pit since it is really a separate object from the street. I guess that the wererats and the ladder would still exist. Perhaps they'd even be able to climb up out of the manhole but if they left the ladder they would need to make a Reflex save to avoid falling into the pit. It seems confusing at best. Perhaps it would be better to rule that you can't cast Create Pit on an open area unless it is "permanently" covered such as by the deck of a ship, not just a hatch or cover. That would be just a house rule to maintain sanity.
@Remy - The manhole situation is interesting. I wonder if the cover wouldn't fall into the Pit since it is really a separate object from the street. I guess that the wererats and the ladder would still exist. Perhaps they'd even be able to climb up out of the manhole but if they left the ladder they would need to make a Reflex save to avoid falling into the pit. It seems confusing at best. Perhaps it would be better to rule that you can't cast Create Pit on an open area unless it is "permanently" covered such as by the deck of a ship, not just a hatch or cover. That would be just a house rule to maintain sanity.
Perfectly valid option.
I like the idea that they could open the manhole and it'd likely then fall into the pit. Then could climb up out of the hole, but it'd be into the middle of the pit, causing them to fall in as well if they tried to fully leave the ladder. They might be able to attempt a jump check to the edge of the pit though.
I like the extra weirdness factor to seeing the effect this way. (taste/preference)