At first glance, the canyon may appear to be a giant crack which suddenly opened up and into which the Yellowstone River then plunged headlong over high waterfalls at its southwest end. This, of course, is not the way the canyon formed. Nevertheless, it is apparent that certain unusual conditions caused the river, after winding slowly through flat-floored Hayden Valley for about 13 miles, to cut a precipitous gorge 1,000-1,500 feet deep and 20 miles long (fig. 42C). A full explanation must be based on all the many events surrounding the eruption of the Yellowstone Tuff, the collapse of the Yellowstone caldera, the outpouring of the Plateau Rhyolite, and the various episodes of glaciation. Geologic studies show that all these events took place while the canyon was being cut, and that each one played an important role in its development. Hot-water and steam activity likewise was a significant factor. However, despite its many complexities, the history of the Grand Canyon can be divided into a few major stages, as outlined below: 1. From more than 2,000,000 years ago to about 600,000 years ago, a shallow canyon was gradually being cut into the Absaroka volcanic sequence by the ancestral Yellowstone River as it eroded headward from a point near the present confluence of the Yellowstone and Lamar Rivers (fig. 33). By the time of the climactic volcanic eruption in central Yellowstone 600,000 years ago, the head of the “old” canyon probably had been eroded southward nearly to the place where the north rim of the Yellowstone caldera was to form later (fig. 42A). This point now lies about 5 miles below Lower Falls. 2. Ash-flow tuffs that were erupted 600,000 years ago filled the “old” canyon, and the river recarved its channel, chiefly along its previous course. 3. A large lake formed behind (south of) the north rim of the caldera, the damming resulting in part from lava flows of Plateau Rhyolite that poured out across the caldera floor in this area between 600,000 and 500,000 years ago. Eventually the lake rose and spilled northward into the head of the “old” canyon, causing additional downcutting in what is now the lower 15-mile stretch of the canyon. 4. As the lake emptied, the river began to erode upstream into the thick rhyolite lava flows toward the present site of Lower Falls; the process was very similar to that of a common stream gully eroding headward into a hillside. At a stage somewhat more than 300,000 years ago, the head of the canyon probably lay near the falls, and the river had cut a channel 400-600 feet deep along this upper 5-mile stretch (fig. 42B).