The Northern Rocky Mountains, usually referred to as the Northern Rockies, are a subdivision of the Canadian Rockies comprising the northern half of the Canadian segment of the Rocky Mountains. While their northward limit is easily defined as the Liard River, which is the northward terminus of the whole Rockies, the southward limit is debatable, although the area of Mount Ovington and Monkman Pass is mentioned in some sources, as south from there are the Continental Ranges, which are the main spine of the Rockies forming the boundary between British Columbia and Alberta. Some use the term to mean only the area north of Lake Williston (the Peace River), and in reference to Northern Rocky Mountains Provincial Park, while others consider the term to extend all the way south, beyond the limit of the Hart Ranges at Mount Ovington, to McBride and Mount Robson.