EROSIOXAL HISTORY OF DRIFTLESS AREA 13 



first cycle and the lower portion in the second cycle of 

 erosion. 



There are, however, distinct limitations to the use of in- 

 terrupted profiles as criteria of rejuvenated streams. Any 

 stream which flows across resistant to non-resistant rock, 

 and which has not yet brought the resistant rock to grade, 

 may develop an interrupted profile in a single cycle of 

 erosion. (Fig. 2). If a relatively flat surface emerged 



( i 1 i I / / / r )^-r-T 



Fig. 2. Diagram showing how interrupted profile abc may be developed in a single 

 cycle of erosion with reference to sea level cd, due to inequalities in the hardness 

 of the rock formations. 



from the sea and came to stand distinctly above its sur- 

 roundings, as in the case of a plateau, water would run 

 down the gently sloping summit surface and down the steep 

 ))ordering slope and would acquire an interrupted profile 

 which would last until the lower portion of the stream had 

 worked headward to the very source of drainage. Any 

 region therefore having unequally resistant rock or any 

 flatfish surface bordered by a distinct descent, may have 

 streams with interrupted profiles. 



In the abstract, interrupted profiles in the streams of a 

 region merely suggest that the region may be in the second 

 cycle ; they do not furnish strong evidence, much less proof, 

 of a second cycle. They amount to strong evidence, only 

 after all other possible interpretations have been eliminated 

 by careful study in the field. 



Stream Terraces 

 The uplift of a surface in which a stream has previously 

 reduced its bed to grade and developed a valley flat causes 

 the stream to intrench itself in the flood plain and form 



