Complex sentence: It has been suggested that a debri slope may have stretched far into the gulf, covering the local Quartzite which had likely eroded out only after the present coastline was established around 6,000 years ago.
Split into simpler sentences: It has been suggested that a debri slope may have stretched far into the gulf, covering the Quartzite. The local Quartzite had likely eroded out after the present coastline was established around 6,000 years ago.