Complex sentence: Heritage Books, Inc.: iii. Some sources imply that it was just the width of valley of the Delaware and its surrounding hillsides, other sources stretch for 20--30 miles to the east and west of the river.
Split into simpler sentences: Heritage Books, Inc.: iii. Some sources imply that it was confined to the width of valley of the Delaware and its surrounding hillsides. Other sources define the region as an area extending for 20--30 miles to the east and west of the river.