The Doctor thought a few unkind things about the rock that served as their footing when Rose pointed out how readily it was collapsing. He didn't want to get stuck in a chasm in a cave on an ice planet. Granted, it was better than dying in a dungeon in Cardiff, but still ...
"Just worry about getting beyond the chasm for now, Rose. I'll figure something out once we're clear." He intended, actually, to give their immediate area a decent scan and, if it was safe, collapse the rock that covered the very end of the chasm so there could be no doubt of the danger, and no way forward.