Great Mill Rock was first developed between 1701 and 1707, when John Marsh built the tidal mill there that gave both the islands their names. During the War of 1812, the army built a blockhouse with cannons on the island to deter the British from making their way from Long Island Sound. The blockhouse burned down in 1821. In 1850, John Clark claimed squatter's rights to Great Mill Rock and began selling food and booze to passing boats. A decade later, he sold the island for $40 to Sandy Gibson, who moved his entire household there, right down to the cows and chickens. Three years later, Gibson sold the land to a Charles Leland for $300, but stayed on for several more years as a tenant, catching eels and flounder for food.The Mill Rocks sat amid many other rocks, reefs, and islets with some colorful names among them: Hen and Chickens, Hog's Back, Frying Pan, Bread and Cheese, and Bald-headed Billy. These riverine obstacles made the churning waters in and around Hell Gate notoriously treacherous to navigate. In 1851, the Army Corps of Engineers began a long campaign of blasting these dangerous obstructions out of existence.
October 10, 1885 saw the annihilation of nine-acre Flood Rock by nearly 300,000 pounds of explosives, much of which had been prepared at a facility on Great Mill Rock. "The largest planned explosion prior to the atomic bomb" (a debatable claim), it was reportedly felt about 50 miles away in Princeton, New Jersey. The event was a huge public spectacle, covered extensively in the NY Times. Some excerpts:
The cross streets east of the upper end of Central Park were full of people moving toward the East River. Down town great numbers of people were climbing to the tops of high buildings, for they were sure that they would hear the thunder of the explosion and see the huge sheets of water shooting into the air. All along the East River front every "coign of vantage" was pre-empted early in the day.Check out this photo of the explosion!
The bulk of the crowd, however, assembled on the sides of the abrupt slopes that descend to the river . . . opposite the scene of the explosion. Men, women, children, dogs, and goats mingled in one broad, variegated mass. . . . Hundreds of people gathered on the tops of the big breweries and other tall buildings that loom one above another on the easterly decline of the city. Away up on the tops of chimneys and on the outermost pinnacles of roofs could be seen the irrepressible, never-to-be-left small boy, filled with the American instinct for getting to the top and looking down on the whole business. Trees had their usual load of sightseers and lamp posts were opportunities to be embraced with avidity. Down along the water's edge the masses concentrated into a solid, sinuous wall that wound around among the piers and wharves as far as the eye could reach to north and south. . . .
[Upon the detonation of the explosives by the young daughter of the general in charge of the project:] Away it flew, that viewless spark, to loose three hundred thousand chained demons buried in darkness and the cold, salt waves under the iron rocks. A deep rumble, then a dull boom, like the smothered bursting of a hundred mighty guns far away beyond the blue horizon, rolled across the yellow river. Up, up, and still up into the frightened air soared a great, ghastly, writhing wall of white and silver and gray. Fifty gigantic geysers, linked together by shivering, twisting masses of spray, soared upward, their shining pinnacles, with dome-like summits, looming like shattered floods of molten silver against the azure sky. Three magnificent monuments of solid water sprang far above the rest of the mass, the most westerly of them still rising after all else had begun to fall, till it towered nearly 200 feet in air. To east and west the waters rose, a long blinding sheet of white. Far and wide the great wall spread, defying the human eye to take in its breadth and height and thickness. The contortion of the wreathed waters was like the dumb agony of some stricken thing. . . .
All around the place the water was turned to a dirty brown by the upheaval of the bottom of the river. The foam was still bubbling, nearly 10 minutes after the explosion. Thousands of pieces of wood, mingled with marine weeds and myriads of dead fish, killed by the shock, were floating down into the East River. . . .
Industrious and thrifty fishermen, armed with scoop-nets, ladled dead fish out of the water into their boats and prepared to make to themselves a feast.
Shattered remains of Flood Rock were collected and used to fill in the gap between the Mill Rocks, creating the single island that exists today. Mill Rock was acquired by the Parks Department in 1953; it's seen occasional recreational use since then — perhaps most notably as co-host of the 1969 New York Avant Garde Festival — but is currently off limits to the public.