Complex sentence: During the late sixteenth and early seventeenth centuries the territory of the Attawandaron was within the limits of present day southern Ontario, except for a single population cluster to the east, across the Niagara River near modern day Buffalo, New York.
Split into simpler sentences: During the late sixteenth and early seventeenth centuries the territory of the "Attawandaron" was mostly within the limits of present day southern Ontario. There was a single population cluster to the east, across the Niagara River near modern-day Buffalo, New York.