The northern boundary was originally thought to be roughly parallel to the latitude of the mouth of the Merrimack River, since the river was assumed to flow primarily west. This was found not to be the case and, in 1652, Governor Endicott sent a survey party to locate the northernmost point on the Merrimack. At the point where the Pemigewasset River, the Merrimack's principal tributary, meets the Winnipesaukee River local Indians guided the party to the outlet of Lake Winnipesaukee, incorrectly claiming that as the Merrimack's source. The survey party carved lettering into a rock there (now called Endicott Rock), and its latitude was taken to be the colony's northern boundary. When extended eastward, this line was found to meet the Atlantic near Casco Bay in present-day Maine.