The Falls of Niagara are an illustration of this method. The river flows from Lake Erie through a level country for a few miles, then begins to go faster as the path becomes steeper, and finally plunges over a hard limestone precipice. Beneath the hard limestone (the top step) are softer beds of shale and sandstone. As the water eats into them and removes them, large portions of the face of the limestone precipice on the top fall into the stream below. Thus gradually the Falls of Niagara are eating their way back to Lake Erie, and have been doing so for hundreds of thousands of years. In the process of doing so the Niagara River has cut out below the Falls a gorge which is not less than seven miles long, from two hundred to four hundred yards wide, and from two hundred to three hundred feet deep. There is no reason to doubt that the Niagara gorge has been entirely cut out in this way, and that at first the river fell over cliffs seven miles farther down its course at Queenstown. The amount of rock thus tunnelled would make a rampart about twelve feet high and six feet thick going round the world at the Equator. Still more gigantic are the gorges or caverns of the Colorado and its tributaries in Western America. The Grand Cañon of the Colorado is three hundred miles long, and in some places more than six thousand feet deep. The country traversed by it is a network of deep ravines, at the bottom of which flow the streams that have dug themselves down from the top of the Colorado tableland.