The Orc Chasm appears in The Big Mountains when you reach level 9 and The Council of Loathing sends you to assist Baron Rof L'm Fao. To cross the Chasm, you need a bridge. If you don't have one, you will see the following message:

You approach the Orc Chasm, and see no way to cross it. You make as if to go around it, and are suddenly intercepted by a group of Smut Orcs.

Try as you might, you are unable to get past them. It looks like you're going to have to find a way to get over the chasm, not around it.

When the Chasm has been bridged, you can adventure in The Valley Beyond the Orc Chasm, after seeing the following message:

You approach the Orc Chasm, and whip out your trusty bridge. You place the bridge across the chasm, and the path to the Valley is clear.

References

The Smut Orcs are probably foreshadowings of the XXX pr0n on the far side of the Chasm. But it's surely no coincidence that they are also "scrotums" backwards.
Phonetically, Orc Chasm alludes to the XXX pr0n as well

Anonymous