Plymouth Rock is the traditional landing site for the pilgrims of the Mayflower. Many visitors are underwhelmed at the stone celebrated as America’s birthstone. It is, after all, just a boulder, and originally it weighed in at about 10 tons (~3 cubic meters in size).
Is this even the real Plymouth Rock? Maybe – only the oral history of an old man in 1741 claimed that it was. However, whether real history or legend, the story “took.” And in 1774, it took 20 teams of oxen to drag the thing from the shore to the town square, or half of it anyway, since it broke into two pieces in the attempt. The rock again broke into two pieces in 1834 while being moved to the front lawn of the town museum. The rock has suffered so much touristic vandalism that small chunks of it can be found in suspicious museums (like the Smithsonian) all over the country.
Geologically speaking (hey, this IS the Earth Story), it’s a glacial erratic: that means, it was carried to this site by a glacier of the distant past (estimated to be ~20,000 years ago in this case) and dumped onto the shoreline when the glacier melted away. Petrologically, it is granodiorite (a medium- to large-grained plutonic rock that is just a bit less rich in quartz than a more normal granite) interpreted to be part of the Dedham Granite.
Dedham granite crops out in several plutons around Boston Harbor, and has been zircon dated to ~607 – 630 million years in age (latest Proterozoic). It is interpreted as originating from the partial melting of even older sedimentary protoliths. As plate tectonics go, at the time of its formation, this granite (including what would someday be Plymouth Rock) was part of the supercontinent Pangea; when Pangea split apart ~250 million years ago, the Dedham granite was also split apart, much of it remaining in Africa. Thus, Plymouth Rock can also be considered a geologic pilgrim contributing to the foundation of America.
Today, about half of the boulder that’s supposed to be the original Plymouth Rock is set within an enclosure at Plymouth Rock State Park, and the other half can still be found on Plymouth Harbor in Plymouth Massachusetts.