14 IOWA STUDIES IN NATURAL HISTORY 



terraces. (Fig-. 3). Terraces so formed involve more than 

 one cycle of erosion. 



However, stream terraces are formed in a single cycle 

 of erosion (1) by unequal widening at the levels of un- 

 equally resistant horizontal strata, (2) by the partial re- 

 moval of glacial or fluvio-glacial fills in valleys, (3) by the 



Fig. 3. Diagram showing liow stream terraces may be formed by uplift of the sur- 

 face after a stream has developed a valley flat. The profile abcdfgh is the cross sec- 

 tion of the valley after grade level AA has been reachetl. By uplift a new level was 

 established at BB, the rejuvenated stream cut the new valley cef, and terraces be 

 and fg were formed. 



recession of falls through temporarily graded valley bot- 

 toms, (4) by the removal of dams, such as landslides, lava 

 flows, sand dunes, etc., (5) by renewed erosion of a valley 

 flat, due to increased volume in a pirate stream, (6) by the 

 intrenchment of graded valley bottoms as a stream acquires 

 greater length and more and longer tributaries and comes 

 to drain a larger area, and increases in volume, (7) by a 

 graded stream receiving less load from its head as gradients 

 are lowered, and intrenching itself, (8) by a stream picking 

 up much fine material from its flood plain and dropping 

 less coarse material in its place, (9) by the shifting of 

 meander belts down stream, etc. These are common pro- 

 cesses and all the events in the history of many stream ter- 

 races take place in a single erosion cycle. 



From the above, it is clear that terraces along the streams 

 of a region can seldom be used as adequate evidence of more 

 than one cycle of erosion. On the other hand, if stream 

 terraces can be analyzed and all possibilities can be elimi- 

 nated except the one involving more than one cycle of ero- 

 sion, they might be considered to be more than merely sug- 

 gestive of a second cycle, especially if there are other evi- 

 dences which corroborate the conclusion. 



