Great Brewster Island is one of the outer islands in the Boston Harbor Islands National Recreation Area, situated some 9 miles (14 km) offshore of downtown Boston. The island has a permanent size of 18 acres (73,000 m2), plus an intertidal zone of a further 49 acres (200,000 m2). Unlike the other outer islands, which are low-lying outcroppings of bedrock, Great Brewster is a drumlin which reaches a height of over 100 feet (30 m) above sea level. It has vegetation cover consisting of Apple Trees, Pear Trees, Sumac, , Grasses and Common Reeds, together with a large Gull colony. The birds are aggressive during nesting season and access by humans, which is by private boat only, is discouraged during that time.