Complex sentence: Much of the trail in Pennsylvania was closed in the 1980s because a terrible gypsy moth onslaught had killed much of the oak forest that the trail passed through, and it grew up with brambles, briars and other vegetation to become impassable.
Split into simpler sentences: Much of the trail in Pennsylvania was closed in the 1980s because a gypsy moth onslaught had killed much of the surrounding oak forest. The trail became overgrown with brambles, briars and other vegetation to become impassable.