The name of John Endicott, Governor of Massachusetts Bay, and the initials of Edward Johnson and Simon Willard were inscribed on this rock on August 1, 1652, and it may be the oldest public monument in New England. It marked the presumed headwaters of the Merrimack River and served as a reference point for the colony's northern border under its 1629 charter. The site is always open for recreation. Pets are prohibited and the park is typically unstaffed in the off-season.