(c) 2002 Andrew Alden, licensed to About.com. (fair use policy)
Plymouth Rock is depicted in this souvenir plate from the John Alden Shop in Plymouth, Massachusetts. The boulder's claim to fame is being in the right place at the right time, when the first group of English settlers in New England landed on Cape Cod in 1620. Ever since then, Plymouth Rock has been a famous stone.
Plymouth Rock is a boulder of Dedham Granodiorite, carried to the Plymouth site during the last ice age by the continental glacier and dropped as an erratic, part of its moraine. Granodiorite, a close relative of granite, is usually a tough stone, but boulders subjected to the rigors of a glacier ride can be damaged. The repeated freezing and stress can weaken the rock's joints and create fractures, and existing fractures can be widened. That would explain why Plymouth Rock has broken more than once in the years since 1620. The About New England Guide, Kim Knox Beckius, has an article about Plymouth Rock showing its picture today, a shadow of its former self.