Complex sentence: The tephra is comprised of Mount St. Helens ash that fell in eastern Washington which, based on Carbon-14 dating from other locations, is placed as falling between 13,350 and 14,400 years BCE.
Split into simpler sentences: The tephra is comprised of Mount St. Helens ash that fell in eastern Washington. Based on Carbon-14 dating from other locations, the ash fell between 13,350 and 14,400 years BCE.