Plymouth Rock is a glacial erratic at rest in exotic terrain. When Mayflower, an English merchant ship, approached the rock, in 1620, the rock, like the ship, had recently been somewhere else. Heaven knew where. Some geologists have said that the rock is Laurentian granite, from north of the St. Lawrence River (Loring, 1920). Most American geologists have preferred a provenance closer to home: Cape Ann, for example, north of Boston (Carnegie Institution, l923); or the region of Cohasset, south of Boston (Shimer, 1951); or even the bed of Plymouth Bay (Mather, l9S2). Wherever the boulder came from, it was many times larger in l620 than it is today.

It was also in one piece. In 1774-, the rock was split in two, horizontally, like a bagel. There were those who feared and those who hoped that the break in the rock portended an irreversible rupture between England and the American colonies. If so, the lower half was the Tory half, for it stayed behind, while the upper part was moved from the harbor side to Liberty Pole Square for the specific purpose of stirring up lust for independence. Scarce was independence half a century old when a new portentous split occurred, in the upper, American, rock. It broke, vertically, into two principal parts, shedding fragments to the side. Eventually, the two halves of the upper part were rejoined by common mortar, containing glacial pebbles from countless sources, and the rock as a whole was reconstructed. The upper part was returned to the waterfront, where a thick filling of mortar was slathered on the lower part, and Plymouth Rock—-with its great sutured gash appearing like a surgical scar—was reassembled so that it would be, to whatever extent remained possible, a simulacrum of the landmark that was there in 1620...