To the south of the cape are several small islands, including Spectacle, Pond, Hog and Western, and to the southeast is Deer Isle.

[1] The rockbed of the Cape and adjacent mainland is composed of "a series of volcanics, rhyolitic and andesitic flows, agglomerates, and pyroclastics" intruded by diorite.

[3] Humans have long cultivated blueberries in the region, and its fruit and foliage are eaten by Cape Rosier's black bears, raccoons, foxes, white-tailed deer and birds.

The northeastern part of the Maine is classified in the Downeast Coast ecoregion (a subset of Acadian Plains and Hills) by the Environmental Protection Agency.

[7] On the water, terns, gulls, and cormorants are abundant, and buffleheads, black guillemots, loons, kingfishers and mergansers are also seen regularly.

The climate is classified as humid continental (Köppen: Dfb), with warm and summers, and long, cold and snowy winters.

[12] People living on the coast ate seafood such as clams, mussels, and fish, and may have hunted marine mammals such as seals.

[16][7] Based on archaeological evidence from the nearby Blue Hill peninsula, the area's inhabitants as of the 12th century were part of a trade network extending far north and south along the Atlantic coast.

[20] Traditionally, Penobscot and other coastal peoples hunted, fished, and grew crops like maize (corn), squash, and climbing beans.

It is disputed whether or not Vikings came as far south as Maine in their travels to North America from Greenland, although it is known that they encountered indigenous people on the coast in the 11th or 12th century.

One historian noted that place names of Algonquian language origin in Hancock and Washington counties "frequently suggest great struggles.

Called Oolaghesee or "the entrails," it is supposedly the remains of a moose calf killed in ancient times by the legendary ... hero Glooscap.

[29] According to an 1886 history, A report of the voyage, written by James Rosier was published soon after the Englishmen returned from their expedition, bringing with them five captives from the Penobscot region.

Rosier's pamphlet described the physical resources available to settlers on the islands and coast of Maine (harbors, rivers, soil, trees, wild fruit and vegetables, and so forth).

[30][33] Most who settled on Cape Rosier in the 1700s were of English descent, and most were farmers or fishermen, along with boatbuilders, some of them employed across the water in Castine, Maine.

[21] Around the era of the mining frenzy in the Western United States, in 1880 a copper vein was found near Goose Falls, a tidal estuary a quarter-mile southwest of Harborside (at 44° 21' N., 68° 48.5' W.).

[39] The mine at Goose Falls produced 10,000 tons of crude ore between July 1881 and September 1883, containing 20% zinc, 3% copper and some lead.

[39] The Maine Legislature passed laws that permitted two dams to be constructed in 1967, which made it possible to drain the Goose Pond estuary for open pit mining.

[39] The mine was designated a Superfund site in 2001, under a federal environmental remediation program to clean up highly polluted tracts of land managed by the EPA.

[40][41][42] In 1954, the couple published Living the Good Life, a book widely read and influential among young Americans of the 1960s and 1970s back-to-the-land movement.

[46] Tourism has been a source of income for the Cape's inhabitants, particularly as a summer destination for vacationers and seasonal residents, since the late 19th century.

[27][47] There is a 1,345 acres (5.44 km2) nature preserve at Holbrook Island Sanctuary State Park containing several coastal ecosystems, including upland forest and meadows, ponds, wetland marshes, and rocky coastline.

Harborside's tabulation area encompasses the cape to the west of Holbrook Island Sanctuary and the end of Weir Cove, as of 2020.