Complex sentence: For many years the portage was an important route from Lower Canada to the interior of the North American continent, and became part of the boundary between British North America and the United States following the American Revolution and treaties delineating the border.
Split into simpler sentences: For many years the portage was part of an important route from Lower Canada to the interior of the North American continent. It became part of the boundary between British North America and the United States following the American Revolution and treaties delineating the border.