The island is named for the wild beach plum shrubs that grow on its dunes, but is also famous for the purple sands at high tide, which derive their color from tiny crystals of pink pyrope garnet.

Before it was an island: On the east is an Ile two or three leagues in length; the one halfe, plain morish grass fit for pasture, with many faire high groves of mulberry trees gardens; and there is also Oaks, Pines and other woods to make this place an excellent habitation, being a good and safe harbor.

[1]"Morish" is now "marsh", and the high gardens of "mulberry" trees may be beach plum, which prefers the crowns of the dunes, although today can be seen on only a few.

The map shows an imaginary English town (the insertion of Charles I of England) of the then-future "South Hampton", about where Newbury is.

After a trial before justices appointed for the purpose, the General Court decided it could not honor the claim, as no one then knew the location of the Naumkeag River, and in any case Mason's grant had been included in another.

It did assess a nominal quit-rent fee of a few shillings on land-holders undeniably within the tract; that is, as far south as Ipswich.

In 1639 a third group from England was granted the remaining land between Newbury and Ipswich and incorporated Rowley, Massachusetts.

There is no official record of the use of Plum Island until then, although Smith's glowing report had included the marsh grass, an enticing feature for herdsmen.

A document of January 2, 1639, survives, however, by which Robert Wallis and Thomas Manning of Ipswich agree to maintain a common herd of 48 hogs on Plum Island starting on April 10 and running to the end of the harvest.

The petition was denied on March 13 with the proviso that Ipswich, Rowley and Newbury were allowed use of the island, which became a pasture for hogs, cattle and horses.

[5] These arguments did not prevail; on October 17, 1649, the court finalized its temporary decision, apportioning 2/5 of the island to Newbury, 2/5 to Ipswich and 1/5 to Rowley.

The name is apparently of local origin; the journal of Margaret Smith (1678–9) relates: Leaving on our right hand Plum Island (so-called on account of the rare Plums which do grow upon it), we struck into the open Sea....[7]In 1776, during the American Revolutionary War, Fort Philip was built on the northern point of the island to defend the Merrimack River.

[8][9] An historian of the region, Joshua Coffin, said of it in 1845: Plum Island, a wild and fantastical sand beach, is thrown up by the joint power of winds and waves into the thousand wanton figures of a snow drift.

Now situated in Essex County, Plum Island is divided among four cities and towns: Newburyport, Newbury, Rowley and Ipswich.

On the mainland opposite, the Massachusetts Audubon Society operates the Joppa Flats Education Center and Wildlife Sanctuary.

Along it are numbered parking lots with boardwalks leading to the beach, overlooks and trails, and facilities for the maintenance of the refuge.

Today, there are numerous lodging options for tourists, including bed and breakfasts, inns, and rental cottages.

The Labrador Current flows from north to south along the shore, migrating sand in that direction and chilling the coastal waters.

Several breakwaters have been constructed along the north coast of the island to protect the beach and impede the process.

The migrating sand moves the outlet of the Merrimack River, which has been artificially fixed at its current location.

Soil Conservation Service planted several thousand black pines, a hardy alpine tree, to help hold the sand.

Plum Island Sound is on a migratory route for many varieties of birds, as well as being a nesting area for piping plovers.

Several prepared observation posts of birds are usually populated by birders with equipment ranging from simple binoculars to expensive telephoto cameras.

They are the greater yellowlegs, mallard duck, least sandpiper, great egret, snowy egret, herring gull, great black-backed gull, osprey, Canada goose, tree swallow, gray catbird, killdeer, glossy ibis, red-winged blackbird, northern mockingbird, least tern, piping plover and peregrine falcon.

Before insect control they swarmed the beach and dunes so thickly as to make human presence there difficult if not impossible from June through September.