EROSIONAL HISTORY OF DRIFTLESS AREA 33 



Intermediate plains might be remnants of a plain of 

 marine erosion, but the erosion remnants above it should 

 be isle-like and its surface should contain marine deposits. 

 Obviously no intermediate plain of great extent could be 

 an original marine plain of deposition. Neither is it clear 

 that the remnants of an intermediate plain could be the sur- 

 face of a series of cuestas formed in a single cycle. If a 

 rough topography were developed by streams and then the 

 lowlands were filled, but not to the level of the summit 

 areas, by glacial material or lava flows having a flat sur- 

 face, streams might so dissect the glacial or volcanic fill as 

 to leave remnants of an intermediate plain which would 

 not record more than one cycle of erosion, in the usual sense 

 at least. 



Fluvial Deposits on Uplands 



Where stream deposits are found occupying topographic 

 positions distinctly above present stream beds, whether 

 they lie on summit areas or areas of intermediate plain, on 

 divides or on slopes above drainage, there is evidence that 

 streams which once deposited, ceased depositing and began 

 to degrade. As old streams most commonly aggrade and 

 young streams degrade their beds, there is suggestion in 

 such relations of deposits to stream beds, that the streams 

 were once old and deposited and became young again, carv- 

 ing out the valleys below the levels of the deposits. That 

 is, such a relationship suggests that the land was uplifted 

 after some portions at least had been brought to grade and 

 that more than one erosion cycle was involved in the history 

 of the topography in which such relationship exists. 



In case of upland or intermediate plains due to structure, 

 streams flowing from soft material to the hard rock which 

 forms the plain may be held at temporary grade on the soft 

 material while degradation of the hard rock is in progress, 

 and might deposit on the upstream side of the hard rock. 

 Later in the same erosion cycle, when the hard formation 

 has been cut through, the streams might cut down, leaving 

 the deposits on the structural plain. It is hardly conceiv- 

 able, however, that deposits which have had such histories 

 would be found widely spread over upland surfaces. 



