Complex sentence: The sandstone and basalt cliffs at Wasson Bluff were formed during the Triassic-Jurassic boundary and they contain a record of the creatures who lived before and after the mass extinction.
Split into simpler sentences: The sandstone and basalt cliffs at Wasson Bluff were formed during the Triassic-Jurassic boundary. They contain fossil remains of the creatures who lived both before and after the mass extinction.