In Maine, the IAT/SIA starts in Katahdin Woods & Waters National Monument in the U.S. State of Maine. The trail heads northeast along the East Branch of the Penobscot River and then east towards Mount Chase and, eventually, Houlton. At Houlton, the trail turns north again and continues to Mars Hill Mountain in Aroostook County. It follows the United States/Canadian international border northward to Fort Fairfield where it crosses into Perth-Andover, New Brunswick.