Complex sentence: The South Coast region, which extends south from Coos Bay to the Oregon-California Border, is distinct from the North and Central Coast regions because of its mountainous nature, due to tectonic uplift and terrane accretion in ancient times.
Split into simpler sentences: The South Coast region, which extends south from about Coos Bay to the Oregon-California Border, is distinct from the North and Central Coast regions due to it's mountainous nature. This is due to tectonic uplift and terrane accretion in ancient times.