The streams which flowed upon the old low-lying peneplain surface meandered sluggishly over deep alluvial or flood-plain deposits, and their courses were little if any determined by the character and structure of the underlying rocks, because, with few exceptions, all rocks were worn down to the general plain level. The uplift of the peneplain, however, caused great revival of activity of erosive power by the streams, the larger ones of which soon cut through the loose superficial alluvial deposits and then into the underlying bedrock. Thus the large, original streams had their courses well determined in the overlying deposits, and when the underlying rocks were reached the same courses had to be pursued entirely without reference to the underlying rock character and structure. Such streams are said to be "superimposed" because they have, so to speak, been let down upon and into the underlying rock masses. As Professor Berkey has well said: "The larger rivers, the great master streams, of the superimposed drainage system, in some cases were so efficient in the corrosion of their channels that the discovery of discordant structures (in the underlying rocks) has not been of sufficient influence to displace them, or reverse them, or even to shift them very far from their original direct course to the sea. They cut directly across mountain ridges because they flowed over the plain out of which these ridges have been carved, and because their own erosive and transporting power have exceeded those of any of their tributaries or neighbors."