A big hole in the ground

It is hard to get a grip on just how big the Grand Canyon actually is.  446km long, up to 29km wide and 1,800 metres deep make for a big hole.  The geology of the canyon is as spectacular as the view but its formation has been the subject of a lot of debate.  It is quite obvious that it has been formed by erosion from the Colorado River, but the rate of erosion has been the topic of much geological debate.  The most recent interpretation of the erosion is that although parts of the canyon formed up to 70 million years ago the whole drainage path of the Colorado developed through the length of the Canyon in the last 6 million years.