The Rivers and Valleys of Pennsylvania.
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rising to an astonishingly even altitude, in spite of their great diversity of structure. Before the existing valleys were exca-
![Fig. 6.](http://upload.wikimedia.org/wikipedia/commons/thumb/6/66/The_National_geographic_magazine%2C_volume_1_-_p257a.png/400px-The_National_geographic_magazine%2C_volume_1_-_p257a.png)
Fig. 6.
![Fig. 7.](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/The_National_geographic_magazine%2C_volume_1_-_p257b.png/400px-The_National_geographic_magazine%2C_volume_1_-_p257b.png)
Fig. 7.
vated, the upland surface must have been an even plain—the Cretaceous baselevel lowland elevated into a plateau. The valleys