Prior to the advance of Pleistocene glaciers into Ohio beginning about 2 million years ago, a major river system flowed from its headwaters in North Carolina northward across Virginia and West Virginia and entered Ohio near Portsmouth. The river, known as the Teays, flowed northward to the vicinity of Chillicothe were it swung northwestward across western Ohio, then westward across Indiana and Illinois, where it joined the ancestral Mississippi River. When an early glacier advanced into Ohio, it blocked the Teays River, forming an ice dam that created a large lake (Lake Tight) in the former river valley in southern Ohio. When Lake Tight eventually began to spill over a low divide, a new drainage system was created, thus ending the long reign of the Teays River. Glacial sediments filled in the old valley of the Teays in western Ohio, but deep-water wells and other data have outlined the course of this buried valley, which is up to 400 feet deep in some areas.