EROSION AL HISTORY OF DRIFTLESS AREA 15 



Intrenched Mecmders 



Meandering streams in young valleys have long been 

 used as evidence of second cycles of erosion in regions. By 

 some they have been used as proof, even in regions which 

 afford practically no other evidence of more than one cycle 

 of erosion\ As commonly interpreted, the history of in- 

 trenched meanders is somewhat as follows: (1) in a first 

 cycle of erosion a stream reaches grade, becomes sluggish, 

 and develops a broad valley flat and broad meanders; (2) 

 the surface is uplifted relative to sea, the stream is re- 

 juvenated, and intrenches itself without changing its mean- 

 dering course. 



The value of intrenched meanders as evidences of more 

 than one cycle depends upon the definition of meanders, and 

 the accuracy with which they may be distinguished from 

 other crooks in streams. During all stages of their history 

 all streams are more or less crooked. The first water which 

 flows over a newly formed land surface concentrates in 

 crooked courses, (1) where there are original depressions 

 irregularly distributed, (2) where unequally resistant 

 materials are not arranged in orderly fashion, or (3) where 

 there are differences in the amount of water supply. If in 

 this stage the streams are flowing over a low, almost flat 

 surface, they are easily deflected and curves are developed 

 which are identical in principle with meanders developed 

 on valley flats by streams at grade. In this first stage of 

 its history such a stream is said to be consequent and its 

 curves might be called consequent crooks. Continuing their 

 histories, such streams lengthen by headward erosion, their 

 heads being extended up the steepest slopes, through the 

 least resistant material, and toward the greatest water 

 supply. Inasmuch as these determining conditions are 

 irregularly distributed on most surfaces, this stage of 

 stream adjustment involves the development of a second 

 set of crooks which are also consequent. As the cycle of 

 valley development and the cycle of land reduction continue, 

 a third stage is reached in which stream piracy takes place. 



1. Gannett, Henry, "Physiographic Types," Folio No. 1, U. S. Geol. Surv. Fourth 

 map and page 2, 



