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The story behind Minnesota’s weirdly shaped northern border

Treaty negotiations based on faulty maps can have odd results.

Map of the Northwest Angle
Map of the Northwest Angle

Minnesota’s Northwest Angle in Lake of the Woods is farther north than any other part of the contiguous United States. Logically, it would seem that this area of about 123 square miles should be in Canada. But this oddest feature of the entire U.S.–Canada boundary was the proper result of American treaties negotiated with Great Britain.

The first step in creating the Northwest Angle was provided by the Treaty of Paris (1783). In recognizing American independence, Great Britain agreed to a U.S.–Canada boundary from the Atlantic Ocean to the Mississippi River. West of Lake Superior the line was to run by middle of lake and stream to the northwesternmost point of Lake of the Woods. From that terminus the specified boundary was a due west line to the Mississippi River.

Although they did not realize it, the American and British diplomats had agreed upon a geographic impossibility. They accepted map maker John Mitchell’s depiction of an egg-shaped Lake of the Woods that had an obvious northwesternmost point. They also believed Mitchell’s claim that a line drawn due west from there would intersect the Mississippi.

John Mitchell’s 1755 map showed Lake of the Woods as roughly egg-shaped.

In 1798, British explorer David Thompson proved that the northernmost source of the Mississippi was south of Lake of the Woods. This discovery confirmed the existence of a northwest boundary gap.

The U.S. obtained an unspecified area west of the Mississippi from France by the Louisiana Purchase. In 1807, American and British diplomats tried to define the northern boundary of Louisiana. The British proposed that the 1783 water line be drawn only to the point where it first touched the forty-ninth parallel. They believed that from that intersection the forty-ninth should be followed westward.

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The American negotiators rejected this proposal. They refused to consider anything that changed the wording of the 1783 treaty. Consequently, the British agreed that the northwesternmost point of Lake of the Woods had to be the starting point of a boundary extension.

The negotiators first provided for a new boundary that ran due north-south. This line was from the northwesternmost point of Lake of the Woods to the forty-ninth parallel. Then the boundary was to follow the forty-ninth westward to the continental divide in the Rocky Mountains. The administration of President Thomas Jefferson refused to approve this agreement because it did not resolve other outstanding issues between the two countries.

The U.S. and Great Britain finally fixed Louisiana Territory’s northern boundary by the Convention of 1818. They accepted the due north-south and forty-ninth parallel lines that had been proposed eleven years earlier. This treaty completed the shaping of the Northwest Angle diplomatically. There was not, however, an accurate map of Lake of the Woods even then. Additionally, no one knew where the forty-ninth parallel was relative to the lake’s northwesternmost point.

The existence of the Northwest Angle was confirmed by a joint American-British surveying commission, which compiled the first detailed map of Lake of the Woods. In 1824, David Thompson, the chief British surveyor, identified the lake’s four possible northwesternmost points. The next year Johann Ludwig Tiarks, an astronomer employed by the British Foreign Office, determined that the lake’s northwesternmost point was at the head of Angle Inlet.

The U.S. and Great Britain accepted Tiarks’ calculations of the northwesternmost point. In the Webster-Ashburton Treaty (1842), they agreed on the boundaries that shaped the Northwest Angle.

Joint American-British boundary commissions determined the angle’s precise boundaries. From 1872 to 1875, surveyors calculated and marked the northwesternmost point and the due south line from it to the forty-ninth parallel. During this survey both Great Britain and Canada tried to eliminate the Northwest Angle by reviving the suggestion made in 1807.

The U.S. government rejected their offers to buy the angle. The Americans realized that it did not have great economic value. But they persisted in rejecting anything that would change the treaty under which they had gained their independence. In 1912, another commission surveyed and monumented the water line boundary from the northwesternmost point.

Since 1925, a joint U.S.–Canada boundary commission has maintained the boundary. Among other things, the commission assures that the boundary is easily identified by appropriate monuments.

For more information on this topic, check out the original entry on MNopedia.