This view is of the Green River valley from an overlook along Green River Bluffs Trail near the Visitor Center. Note how the distant hilltops are all a uniform elevation. The Green River has incised into the Pennyroal Plateau, a part of the ancient landscape surface that extended across the entire midwest region prior to the periods of glaciation starting in the Pliocene Epoch (about 5 million years ago). Prior to the glaciation periods, major drainages in the Midcontinent drained northward into the Hudson Bay region. As continental ice sheets formed through the following Pleistocene Epoch, north-flowing river drainages were blocked, great lakes formed and eventually spilled over their divides, diverting the drainage systems to the Mississippi River valley. Over time the greater Ohio River system formed with headwater streams, like the Green River, began downcarving into their channels. With the passing of each of the major glaciation periods, the Green River incised further into its valley. This set the stage for ongoing cavern development in the Mammoth Cave region.