The Mayflower Drops Anchor in Plymouth Harbor, Location of Plymouth Rock, where Colonists are Said to Have First Set Foot on Land

There are no contemporary references to the Pilgrims' landing on a rock at Plymouth. There are two primary sources written by the Pilgrims themselves describing the landing in Plymouth in 1620, William Bradford's journal Of Plymouth Plantation and the 1622 book popularly known as Mourt's Relation. Both simply say that the Pilgrims landed. Neither mentions any rocks in their account of the landing. The first references to Plymouth Rock are found over 100 years after the actual landing.

http://www.pilgrimhall.org/Rock.htm