Avenue. Harlem/Illinois Route 43 serves as its western border, where between Roosevelt and South Boulevard, it borders Forest Park and between North Boulevard and North Avenue to the west it borders River Forest. The entire village of Oak Park lies on the shore of ancient Lake Chicago, which covered most of the city of Chicago during the last Ice Age, and was the forerunner to today's Lake Michigan. Ridgeland Avenue in eastern Oak Park marks the shoreline of the lake, and was once an actual ridge. As with the geographical setup of the Chicago River, which connects to the present day Lake Michigan just north of the city's Loop, the ancient Des Plaines river once emptied into glacial Lake Chicago, making prehistoric Oak Park a "Plains river Delta" system. One of North America's four continental divides runs through Oak Park. This divide, a slight rise running north–south through the village, separates the Saint Lawrence River watershed from the Mississippi River watershed, and is marked by one plaque on Lake Street at Forest Avenue and another in the northwest corner of Taylor Park. According to the 2010 census, Oak Park has a total area of , all land. Transportation Public transit Oak Park is accessible from Chicago by service on the Green Line and the Blue Line at five CTA stations in Oak Park. Oak Park also has a station for Metra's Union Pacific / West Line. Bus transit service within Oak Park and to other suburbs is also provided by the CTA and Pace. Streets, addresses, and expressways The Eisenhower Expressway is the primary expressway between Chicago and Oak Park. The highway also provides connections to O'Hare International Airport. Major east–west streets in Oak Park continue east into Chicago. The streets are laid out in a grid pattern, occasionally with local streets ending in a cul-de-sac to maintain local character. Oak Park has its own street-numbering system that begins, for east–west streets at Austin Boulevard (no east or west designation), and for north–south streets, at the elevated train tracks located just south of Lake Street, which divides the numbers, getting larger going north or south from there, and requiring north or south designation on addresses. The border streets do not follow the Oak Park numbering system; rather, they match the address system with the cities sharing those border streets. For example, addresses on Austin Boulevard match the Chicago system, with