Complex sentence: They start to get out, and no sign of the "Allosaurus" yet, until Spike gets stuck though the gap in the log, and when he gets out, the "Allosaurus" breaks the log and falls into the ravine below.
Split into simpler sentences: They start to get out, with no sign of the "Allosaurus" yet, until Spike gets stuck in a gap in the log. When he gets out, the "Allosaurus" breaks the log and falls into the ravine below.