Prior to the beginning of the Pleistocene Ice Age about two million years ago, Ohio’s drainage system consisted of at least two major river systems. Best known is the Teays system that entered southern Ohio and swung westward across Indiana and Illinois to the ancestral Mississippi River system. It has been postulated by some geologists that the Lake Erie basin was the site of an eastward-flowing river system that connected with the ancestral St. Lawrence system. This river was named the Erigan River. Unlike the Teays system, where numerous segments of its valley and tributary valleys have been preserved, the Erigan River was destroyed by the advance of multiple glaciers during the Pleistocene. Some geologists have suggested that the Teays River connected to the Erigan River and did not flow across western Ohio and Indiana. Evidence for this is equivocal.