Okay, so while looking up some details for @ScottDeWar_jr in Heroes of Battle, I read some more guidelines about moving on a steep slope. It seems the 2 square penalty is only for moving uphill, not downhill or horizontally.
There is also a rule that states that if anyone tries to run or charge downhill, they must make a DC 10 Balance check. If the check fails, the character stumbles and must end their movement 1d2x5 squares later. If the check fails by 5 or more, the character falls prone.
A steep slope also increases the DC of a Tumble check by 2.
There is also a rule that says a crevasse should be 2d4x10 feet deep, so I'm going to say my 100 ft. crevasses are actually only 80 feet deep. That is still 8d6 damage of falling damage for any bad guys that the PCs manage to push into one of them.