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Some Neglected Principles of Physiography. 



A. H. Purdue. 



All sciences suffei* from errors and misconceptions, whicli have in one 

 way and anotlier crept in ; and as such are difficult to eliminate. Many are 

 passed on from older to younger workers, and are used in both theory and 

 practice. Of such in geology are the popular notions of the characteris- 

 tics of entrenched meanders; the origin of limestone sink-holes; and (in 

 the opinion of the writer) of anticlinal valleys, and possibly of some 

 transverse drainage. 



The Entrenched Meander. It is an accepted principle of physiography 

 that after a stream reaches base-level, it begins to meander. Uninter- 

 rupted by diastro]ihic mo\ement. the meandering continues until the region 

 on either side has been reduced to a plain, the width of which depends 

 upon the size and strength of the stream. If such a region be elevated, 

 the stream, from renewed vigor, will resume the downward cutting of its 

 bed, producing a new (entrenched or incised) valley within the old one. 

 Thus far, the popular notion of the entrenched meander can be accepted 

 without question ; but it is also the popular though erroneous notion that 

 the new valley occupies the bed of the old one, and is V-shaped. 



While it appears that some rejuvenated streams do have V-shaped val- 

 leys, such are rare. The rule is that the valleys of such streams are un- 

 symmetrical. The slopes above the insides of the curves are of compara- 

 tively low gradient, while those on the outside are steep. This may be 

 seen by inspecting almost any good toixigraphical map of an area with 

 rejuvenated streams. The explanation is simple. In an old stream, the 

 downward cutting is little or nothing, while the lateral cutting on the 

 outside of the bends may be relatively great. In rejuvenated streams, the 

 downward cutting is resumed, but the lateral cutting does not cease. On 

 the contrary, it becomes more rapid, because the impingement upon the 

 banks is greater than before. The resulting topography is shown in Fig. 

 1, A and B. As all bends become greater, the rejuvenated stream is more 

 crooked than when in its previous stage of old age, which of course means 

 that it has shifted from its old bed. 



