DRAINAGE PATTERNS (CONTINUED)

A different type of drainage forms on sedimentary rocks that are tilted or folded and that have different susceptibilities to erosion.  In such a situation the more resistant rocks tend to stand as high ridges whereas the more easily eroded softer rocks are cut down to form valleys.  Both the ridges and the valleys are long parallel topographic features, running along the bands where the tilted beds intersect the Earth's surface.  Such topography is called "valley and ridge" topography.  The province of that name in northwest Georgia has such a landscape.

The map below is a 1:50,000 scale map of a part of eastern West Virginia near Franklin, and shows the type of drainage that typically develops on valley and ridge topography.  Not surprisingly, the trunk streams (Root Run, Seneca Creek, and both branches of the North Fork of the Potomac) flow along the axes of the valleys.  On this map they all drain toward the north.  Their tributaries are short, are generally unbranched themselves, and run almost directly down the sides of the adjacent ridges.  This is most obviously seen in the tributaries of Seneca Creek and the North Branch.  Clearly bedrock geology controls where the streams can flow, and thus the drainage pattern they must follow.

If you re-examine the map on the previous page you will be able to see that northwestern Georgia has a similar drainage pattern.  The valleys are broader in Georgia and the pattern harder to illustrate on a smaller scale map.

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