Immediately west are Primera Angostura and Segunda Angostura, narrows formed by two terminal moraines of different ages.[46] The Primera Angostura is the closest approach of Isla Grande de Tierra del Fuego to mainland South America. Farther west lies Magdalena Island, part of Los Pingüinos Natural Monument. The strait's southern boundary in the east follows first the shoreline of the Isla Grande de Tierra del Fuego, then the northern end of the Canal Whiteside and the shoreline of Dawson Island.