Ian's extra weight on the rope bridge causes the "cave wall" scenery, to which it is tethered, to bend.
The icicles are made of polystyrene - this is revealed by the unpainted end of one of the icicles that Ian uses to construct a bridge.
It is so cold that Ian and Barbara nearly die. Everyone has to wear furs or risk freezing. So how come Altos can wander around without any trousers on? Even given his impressively muscular legs, he must be a bit chilly.