The Delaware River was often referred to as the Minisink River in early Dutch colonial documents and on early maps. The Delaware River constitutes part of the boundary between Pennsylvania and New York, and the entire boundary between New Jersey and Pennsylvania. The middle section of the Delaware River's course, roughly between Port Jervis, New York and the Delaware Water Gap (a break in Kittatinny Mountain where the river runs through) constitutes the north and southern points of the Minisink or Minisink Valley. The river flows down a broad Appalachian valley. The Minisink is a buried valley, where the Delaware flows in a bed of glacial till that buried the eroded bedrock during the last glacial period.