Page:Popular Science Monthly Volume 46.djvu/235

Rh by the cleaning out of the clogged valleys. A pretty contrast for our geographical lessons!

It is noticeable that the abandoned channels of the glacial lakes are now generally utilized as natural paths of communication. The Indians easily passed from the head of the Minnesota to the Red River of the North; and indeed at times of high water they paddled their canoes over the flat divide. The Chicago outlet of Lake Michigan was naturally chosen for the path of the Illinois and Michigan Canal, and now two railroads follow the same well-graded course. More important still is the broad channel into which the Mohawk flows from the Adirondacks, and which we therefore call the Mohawk Valley. This well-opened passage determined the location of the Erie Canal; and that, taken with the drowning of the Hudson River, by which navigable tidewater is carried up to Albany, has undoubtedly been the determining cause in the development of New York city as our greatest seaport.

As a good number of these abandoned channels have been described, and as it is very probable that others will be found, it seems to me worth while to recognize them as constituting a special group of river-made forms of brief and peculiar history, deserving recognition and representation in our study of physiography. In this connection a particular interest attaches to the former outlet of Lake Michigan, because it is the only one of the old outlets that is now mapped with any approach to accuracy. Twelve sheets prepared by the United States Geological Survey—namely, the Chicago, Riverside, Calumet, Desplaines, Joliet, Wilmington, Morris, Ottawa, Marseilles, Lasalle, Hennepin, and Lacon sheets—already represent a length of over a hundred miles of the former lake outlet, and give an efliective illustration of its peculiar features.

Before speaking of the old channel, I must turn from that theme to give the maps a fuller introduction, for they seem as yet to be very little known to our teachers and scholars. They are constructed to serve as the topographical basis for the geological map which our national survey is charged to prepare. No suitable map existing. Major Powell, director of the survey, organized a topographical corps in 1882, under the charge of Mr. Henry Gannett, to whom the mapping of the country was intrusted. The progress of the work has manifestly been embarrassed by the expense of the survey over so vast a country, by the need of comparatively rapid progress, and by the difficulty of securing experienced topographers; but all considerations of distribution, scale, cost, and time have been duly discussed, and as a result we have already several hundred map sheets of areas distributed over many States, on which the topographic features of