Complex sentence: Geologists believe the canyon was likely formed thousands of years ago when sea levels were much lower during a cooler climate period associated with the most recent ice age, as the Hudson River emptied into Atlantic Ocean hundreds of miles further off shore than the present shoreline.
Split into simpler sentences: Geologists believe the canyon was likely formed thousands of years ago during a cooler climate period associated with the most recent ice age. As sea levels were much lower then, the Hudson River emptied into Atlantic Ocean hundreds of miles further off shore than the present shoreline.