In the north, the Sundance Sea, an extension of the Arctic Ocean, stretched through Canada down to the United States. Coal is found in the Morrison Formation of Montana, which means that the northern part of the formation, along the shores of the sea, was wet and swampy, with more vegetation. Aeolian, or wind-deposited sandstones, are found in the southwestern part, which indicates it was much more arid—a desert, with sand dunes.