Five Michigan Upper Peninsula counties include nearby major islands: Mackinac Island, Round Island and Bois Blanc Island in Lake Huron are in Mackinac County; Sugar Island and Neebish Island in the St. Marys River, and Drummond Island in Lake Huron are in Chippewa County; Grand Island is in Alger County; Summer Island is Delta County; and Isle Royale is part of Keweenaw County.The peninsula is divided between the flat, swampy areas in the east, part of the Great Lakes Plain, and the steeper, more rugged western half, called the Superior Upland, part of the Canadian Shield. The rock in the western portion is the result of volcanic eruptions and is estimated to be at least 3.5 billion years old (much older than the eastern portion) and contains the region's ore resources. Banded-iron formations were deposited 2 billion years ago; this is the Marquette Range Supergroup. A considerable amount of bedrock is visible. Mount Arvon, the highest point in Michigan, is found in the region, as well as the Porcupine and Huron mountains. All of the higher areas are the remnants of ancient peaks, worn down over millions of years by erosion and glaciers.