ORIGIN OF TOPOGRAPHIC FORMS. H3 



to a level with the main stream. The same operation which is going on in 

 the main stream is going on in these affluents, but with different intensity, 

 owing to their smaller volume of water and perhaps smaller amount of sedi- 

 ment, and to the fact noted below, that their mouths are constantly being 

 lowered. Now, following up these branches as they subdivide into smaller 

 and smaller streams, a point is finally reached where the little rivulets, with 

 their feeble cutting power, are only able to keep their lower courses cut 

 down to the level of the stream to which they are tributary. They have 

 no energy to spare in working back up their own courses. At this point the 

 curve changes from one concave upward to one convex upward. This con- 

 vex curve is the curve of all the minor rain-water rivulets — in short, it is the 

 curve of the terrain — while the concave curve is the curve of the water 

 courses. The former is the curve of the upper relief of the country, the 

 latter is the curve of the valleys. 



The relative extent of these two curves depends mainly upon the 

 climate and upon the range of elevation of the country, ox - , in other words, 

 upon the relative rapidity of corrasion of their beds by the perennial streams, 

 and the erosion of the terrain by the rain-water rivulets. In a well- watered 

 region, where the range of elevation is small, and where the larger streams 

 are near base level, the hill forms are broad, rounded, and convex, and the 

 valleys are equally rounded, with concave forms. Of this type is the undu- 

 lating billowy surface of the Great Plains ami the Atlantic and Gulf plains 

 of the Southern states. 



Where the range of elevation is great, the curves both of valley and 

 ridge become sharper and more angular. The streams have a greater fall 

 and proportionally increased power, and therefore cut more rapidly; but, 

 on the other hand, they have more work to perform. The Cumberland 

 plateau, with its intricate network of streams, consists of a close alternation 

 of ridges and valleys, with straight slopes at very steep angles, the bottoms 

 of the gorges and the summits of the ridges being but slightly rounded. 

 Few of the streams have reached base level, except in some cases near their 

 mouths, and corrasion of their beds is still active. In a high mountain range 

 all these features become still more accented. The main streams have a 

 steep descent and corrade their beds rapidly. Their valleys are narrow, 



