Paris is located on a loop of the Seine, between its confluences with the Marne and the Oise, in the heart of a vast sedimentary basin called the Parisian basin. The city extends on both sides of the river around its historic heart embodied by the two islands of the Seine: the Île Saint-Louis to the east and the Île de la Cité to the west. Two-thirds of its 65.6 sq. miles (105.40 km2) surface area is located to the north of the river, on the right bank, and one-third to the south, on the left bank. Paris intra-muros is separated from its neighbouring communes by the artificial border of its ring road, an urban expressway a 28 miles (34.98 km) long.