The glaciers of the last Ice Age retreated north about 25,000 years ago, leaving the lakes that became Michigan's most notable geographical feature. Among the state's largest inland lakes is Burt Lake, named for William A. Burt, who with John Mullett made a federal survey of the area from 1840 to 1843. By following the Cheboygan River, Mullett Lake, and Indian River to Burt Lake, Indians and fur traders needed only a short portage to Little Traverse Bay and avoided the trip through the Straits. Completion of a lock on the Cheboygan in 1869 opened this inland waterway to the Cheboygan Slack Water Navigation Company, whose vessels carried passengers and freight until railroads put it out of business. Day-long excursions over these waters later became popular with tourists.