There may have existed something of a lake in preglacial times, through which the river ran, but it undoubtedly owes its present width to the grinding action of the irresistible icebergs and the piling up of débris on the shores. The river bed was filled up by a glacial drift at the point of its present outlet until the lake was raised in its level much higher than that of Lake Michigan. Another glacier plowed down through Lake Michigan, widening it out to its present dimensions, while the glacial drift was deposited at what is now the head of the lake, filling up the old outlet and thus making a great dam. The damming up of these great water courses was another cause for increasing the width of these lakes. In a similar way Lake Erie was formed. It is supposed, however, that this lake is entirely the product of glacial action, as there is no evidence of an old river bed in its bottom; besides, it is much shallower than the other lakes. The same action that formed Lake Erie filled up the old river bed running through the province of Ontario, so that when the ice receded Lake Erie became the new channel for the old river. The same process filled up the Valley of the Mohawk to more than 100 feet in depth and also raised the Valley of the Hudson. This caused the new channel to be made through the Niagara River and a new route to the ocean for the drainage of all the chain of lakes through the St. Lawrence. It will be seen that the bottoms of all of these great lakes to a certain extent were worn out by the action of running water, except Erie. The great glaciers widened them out, and in the case of Lake Erie scooped it out. At the same time it built great dams across the outlets which raised the surface of the water to a much higher level and caused them to form new outlets, thus changing the whole face of the country over which the ice drifted. The glaciated region of North America is among the most productive in the world, and in many respects presents a most pleasing landscape. Other lakes besides these mentioned have been formed during the ice period through blocking the course of a river by the ice itself. Dr. Wright, during the time he traced out the line of the terminal moraine, discovered that the ice sheet crossed the Ohio River at a point near Cincinnati, where there is a great bend to the northward in the river. With the exception of this point and perhaps another point below, the edge of the great ice sheet kept a little north of the Ohio River. At this point, however, the ice seems to have filled the valley from hill to hill, which very naturally would form a great dam or lake in the Ohio Valley. Of course such a lake could not be permanent, because, when the ice melted away, it again opened the channel and allowed the water to flow off.