=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. [Illustration: 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?] =Lakes.= Lakes are perhaps the most obvious of these fleeting features of infancy. They are short-lived, for their destruction is soon accomplished by several means. As a river system advances toward maturity the deepening and extending valleys of the tributaries lower the ground-water surface and invade the undrained depressions of the region. Lakes having outlets are drained away as their basin rims are cut down by the outflowing streams,--a slow process where the rim is of hard rock, but a rapid one where it is of soft material such as glacial drift. Lakes are effaced also by the filling of their basins. Inflowing streams and the wash of rains bring in waste. Waves abrade the shore and strew the débris worn from it over the lake bed. Shallow lakes are often filled with organic matter from decaying vegetation. Does the outflowing stream, from a lake carry sediment? How does this fact affect its erosive power on hard rock? on loose material?