Other proofs of the same movement are drowned land forms which are modeled only in open air.  Since rivers cannot cut their valleys farther below the baselevel of the sea than the depths of their channels, drowned valleys are among the plainest proofs of depression.  To this class belong Narragansett, Delaware, Chesapeake, Mobile, and San Francisco bays, and many other similar drowned valleys along the coasts of the United States.  Less conspicuous are the submarine channels which, as soundings show, extend from the mouths of a number of rivers some distance out to sea.  Such is the submerged channel which reaches from New York Bay southeast to the edge of the continental shelf, and which is supposed to have been cut by the Hudson River when this part of the shelf was a coastal plain.

Warping.  In a region undergoing changes of level the rate of movement commonly varies in different parts.  Portions of an area may be rising or sinking, while adjacent portions are stationary or moving in the opposite direction.  In this way a land surface becomes warped. Thus, while Nova Scotia and New Brunswick are now rising from the level of the sea, Prince Edward Island and Cape Breton Island are sinking, and the sea now flows over the site of the famous old town of Louisburg destroyed in 1758.

Since the close of the glacial epoch the coasts of Newfoundland and Labrador have risen hundreds of feet, but the rate of emergence has not been uniform.  The old strand line, which stands at five hundred and seventy-five feet above tide at St. John’s, Newfoundland, declines to two hundred and fifty feet near the northern point of Labrador.

The great lakes is now under-going perceptible warping.  Rivers enter the lakes from the south and west with sluggish currents and deep channels resembling the estuaries of drowned rivers; while those that enter from opposite directions are swift and shallow.  At the western end of Lake Erie are found submerged caves containing stalactites, and old meadows and forest grounds are now under water.  It is thus seen that the water of the lakes is rising along their southwestern shores, while from their north-eastern shores it is being withdrawn.  The region of the Great Lakes is therefore warping; it is rising in the northeast as compared with the southwest.

From old bench marks and records of lake levels it has been estimated that the rate of warping amounts to five inches a century for every one hundred miles.  It is calculated that the water of Lake Michigan is rising at Chicago at the rate of nine or ten inches per century.  The divide at this point between the tributaries of the Mississippi and Lake Michigan is but eight feet above the mean stage of the lake.  If the canting of the region continues at its present rate, in a thousand years the waters of the lake will here overflow the divide.  In three thousand five hundred years all the lakes except Ontario will discharge by this outlet, via the Illinois and Mississippi rivers, into the Gulf of Mexico.  The present outlet by the Niagara River will be left dry, and the divide between the St. Lawrence and the Mississippi systems will have shifted from Chicago to the vicinity of Buffalo.