Complex sentence: Some estimates say that the coastline of New England used to extend 75 miles east of its current position; a Native American of the era could have walked from Nantucket to southern Cape Cod without touching the Atlantic Ocean.
Split into simpler sentences: Some estimates say that the coast line of New England used to extend 75 miles east of its current position. That would mean a Native American of the area could've walked from Nantucket to southern Cape Cod, without touching the Atlantic Ocean.