In 1820, Cass led an expedition to the northwestern part of Michigan Territory, in the Great Lakes region in today's northern Minnesota. Its purpose was to map the region and locate the source of the Mississippi River. The headwater of the great river was then unknown, resulting in an undefined border between the United States and British North America, which had been linked to the river. The Cass expedition erroneously identified what became known as Cass Lake as the Mississippi's source. It was not until 1832 that Henry Rowe Schoolcraft, the Cass expedition's geologist, identified nearby Lake Itasca as the headwater of the Mississippi.