Fig. 45. A Young River, Iowa
Note that it has hardly begun to cut in the plain of glacial drift on which it flows
Contours. In order to read the topographic maps of the text-book and the laboratory the student should know that contours are lines drawn on maps to represent relief, all points on any given contour being of equal height above sea level. The contour interval is the uniform vertical distance between two adjacent contours and varies on different maps. To express regions of faint relief a contour interval of ten or twenty feet is commonly selected; while in mountainous regions a contour interval of two hundred and fifty, five hundred, or even one thousand feet may be necessary in order that the contours may not be too crowded for easy reading.
Whether a river begins its life on a lake plain, as in the example just cited, or upon a coastal plain lifted from beneath the sea or on a spread of glacial drift left by the retreat of continental ice sheets, such as covers much of Canada and the northeastern parts of the United States, its infantile stage presents the same characteristic features,—a narrow and shallow valley, with undeveloped tributaries and undrained interstream areas. Ground water stands high, and, exuding in the undrained initial depressions, forms marshes and lakes.
Fig. 46. A Young Drift Region in Wisconsin
Describe this area. How high are the hills? Are they such in form and position as would be left by stream erosion? Consult a map of the entire state and notice that the Fox River finds its way to Lake Michigan, while the Wisconsin empties into the Mississippi. Describe that portion of the divide here shown between the Mississippi and the St. Lawrence systems. Which is the larger river, the Wisconsin or the Fox? Other things being equal, which may be expected to deepen its bed the more rapidly? What changes are likely to occur when one of these rivers comes to flow at a lower level than the other? Why have not these changes occurred already?