Porthmadog lies in Eifionydd, on the estuary of the Afon Glaslyn, where it runs into Tremadog Bay. The estuary, filled with sediment deposited by rivers emptying from the melting glaciers at the end of the last Ice Age, is a haven for migrating birds. Oystercatchers, redshanks and curlews are common, as are summer flocks of sandwich terns. To the west looms Moel y Gest, rising above the town as a Marilyn.