46 IOWA STUDIES IN NATURAL HISTORY 



tion to rock formations, with this statement, the writer con- 

 siders it best to continue the custom. 



Various methods may be used in determining the periods 

 or epochs to which certain raised peneplains belong, the 

 method used depending upon the conditions existing in the 

 region under investigation. Some of these methods are 

 here mentioned: (1) Any graded erosion surface is 

 younger than the youngest formation which it cuts, and 

 (2) younger than any structure it bevels. The youngest 

 system forming the oldest peneplain surface in the Appa- 

 lachian mountains is the Pennsylvanian, and the folds and 

 faults across which the surface is developed took place in 

 the Permian, The peneplain is therefore not only post- 

 Pennsylvanian, but is post-Permian. (3) An old erosion 

 surface is younger than any formation of which there are 

 distinguishable fragments or fossils in fluvial deposits on 

 the surface. This is illustrated in the Driftless Area where 

 stream gravels containing chert pebbles and fossils of 

 Niagaran age lie on divides where the uppermost rock is 

 pre-Niagaran ; the divides must be remnants of a surface 

 which is at least younger than mid-Silurian. (4) Pene- 

 plains are contemporaneous with fluvial deposits which lie 

 on them, (5) contemporaneous with or older than other 

 terrestrial deposits lying on them, and (6) older than 

 marine formations lying on them. Peneplains are (7) old- 

 er than valleys which have been cut below them. An old 

 peneplain is (8) younger than rocks forming erosion rem- 

 nants above the plain and (9) older than deposits in valleys 

 below it. A peneplain is (10) younger than any adjacent 

 peneplain which stands at a higher level and (11) older 

 than any lower adjacent graded plain. In the case where 

 subsidence has taken place and streams have been caused 

 to develop grades at levels higher than was possible before 

 subsidence occurred, points (10) and (11) would be re- 

 versed. The higher of two graded surfaces in this case 

 would be the younger. The lower one would be buried and 

 would only in the rarest case be distinguishable. (12) If 

 an erosion surface has been uplifted by tilting, warping, 

 folding or faulting, and there are deposits which have not 



