Complex sentence: When the lava cooled down and solidified everything that were left from the trees were "cylindrical hollows" or tree molds that are commonly created in Pāhoehoe lava flows and could be seen along Devastation Trail.
Split into simpler sentences: When the lava cooled down and solidified, all that was left of the trees were hollows cylinders known as tree molds. These are commonly created in Pāhoehoe lava flows, and can be seen along Devastation Trail.