Plymouth Rock is an American icon, a symbol of intrepid discovery, liberty, and freedom of conscience. The stone itself is granite, probably from a formation known as the Dedham granite, formed 608 million years ago (give or take 17 million years) - that is, a long time before 1620!

The Rock was picked up from this formation at a spot south or west of Boston and transported by a glacier to Plymouth about 20,000 years ago. The spot it left was somewhere in the terrane (specific geologic area) called Atlantica, which surrounds Boston.

When the Pilgrims arrived, they may or may not have stepped on Plymouth Rock. If they did, they never mention it in their letters and written accounts. In any case, the Rock was much larger in 1620, but erosion by sea and wind has reduced it to a mere fraction of its former self.

Nature did havoc to the Rock, but humans did worse, chipping off small pieces for patriotic souvenirs, taking large pieces to put on display to build patriotic fervor, even using it as part of a wharf at one time. In 1774, 20 yoke of oxen came to move Plymouth Rock, and it split in the process. Half of the Rock was put on display at Pilgrim Hall Museum from 1834 to 1867, but was then brought back here.

In November 1989, the Rock was repaired and strengthened to withstand the blows from the sea and tossed coins of a million affectionate visitors. Today Plymouth Rock is sheltered by a monumental enclosure, designed by McKim, Mead & White and built in 1921, which stands in Massachusetts' Plymouth Rock State Park.