Complex sentence: The petroglyph boulder occurs within an artificially created bowl-shaped depression, which is currently covered with mowed grass (previously a corn field) and bordered on the west by a thicket of river cane (Arundinaria gigantea).
Split into simpler sentences: The petroglyph boulder occurs within an artificially created bowl-shaped depression. Today this is covered with mowed grass (it was previously cultivated as a corn field) and bordered on the west by a thicket of river cane (Arundinaria gigantea).