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One of the most important jobs for a GM when developing a campaign is worldbuilding. And when you think about it, the fundamental bedrock (literally) of worldbuilding is geology. If you know a bit about how landforms are made, how erosion occurs, how landscapes change over time, that will take you a long way to developing and establishing a world that is at least realistic enough to be convincing.
All the planetary sciences are helpful: geology, geography, weather and climate. And also fun!
Commentary by memnarch (who has not seen the movie)
Or we can wait a bit longer to find out if Kylo and Luke get shot at by AT-ATs, I suppose. A little odd for the movie to cut at that spot perhaps, but then again, maybe it didn't really cut at this point. Or it's to show that Luke is managing to stall Kylo for a long period of time. They're already into some tunnels away from the front door, so that's gotta be at least a few minutes' walk.
Wouldn't limestone need something to make it erode and collapse though? If this was Earth, I'd say water would do the trick quite handily, but we've established that that doesn't exist on this planet to any meaningful degree now. I'm sure Jim would have some proper answer to that, like maybe the collapses happened during the big evaporation, but it looks like he and his character are suddenly busy trying to dig themselves out of this cave-in they made.
Anyway, I'll guess that Chewie and Rey figure something out to get the boulders out of the way. They seem to be the most competent characters recently besides Luke, and he's also busy keeping the solicitors at the front door.
Transcript
Poe: C’mon! We need to get out of here before Kylo makes mincemeat of Luke!
GM: You race through the salt tunnels.
Poe: It won’t be salts at this elevation above the evaporated sea bed.
GM: Oh?
Poe: Ancient seas would have been prime habitat for creatures with carbonate shells.
Poe: Tectonic forces would compress them into limestone and raise them to form these mountains, which remained above the salt when the seas dried up.
GM: Right. Limestone tunnels.
Poe: But limestone is prone to erosion and collapse. It’ll be a miracle if this escape route hasn’t caved in.
{beat, we see the tunnel ahead of them has caved in}
GM: One day I’m going to write a book about gamemastering, and the first thing it’ll say is “Learn geology”.