48 IOWA STUDIES IN NATURAL HISTORY 



least from the ages of the different erosional surfaces. For 

 instance, if it has been proven that an upland peneplain in 

 a given district is Cretaceous in age and there is an inter- 

 mediate plain below it which is Eocene, it is a short and 

 simple step to the conclusion that the uplift of the upper 

 plain took place at or near the close of the Cretaceous 

 period. But if two consecutive erosion surfaces are more 

 widely different in age, as late Cretaceous and early Pleis- 

 tocene, the uplift of the Cretaceous plain may have taken 

 place at any time between the two periods ; that is at the 

 end of the Cretaceous, or during or at the end of the 

 Eocene, Oligocene, Miocene or Pliocene. 



In such cases as the last the student is likely to fall back 

 on an estimate of the amount of time it must have taken 

 to produce the second plain after the uplift of the first. If 

 it seems that it would have required the Miocene and Pli- 

 ocene periods to produce the lower plain, it might be as- 

 sumed that the uplift took place at the end of the Oligocene, 

 but this assumption would not be without possibility of 

 serious error. If the conclusion was reached that the up- 

 lift of the older plain did take place at the close of the 

 Oligocene, this plain would probably thereafter be called 

 the Oligocene rather than the Cretaceous plain, for the 

 period name given it would be that designating the latest 

 period at which the plain is believed to have been intact. 



The conclusion arrived at is that the dates of diastrophic 

 movement can be told in a general way from the ages of 

 consecutive erosion surfaces, but that the closer together 

 the surfaces are in age the more accurately can the date of 

 the diastrophism be determined. 



Duration of Geologic Time 

 A rough estimate of the duration of certain geologic 

 periods might be made if the ages of consecutive erosion 

 surfaces and the dates of uplift are known. For instance, 

 if an upland plain with remnants at an average altitude of 

 2,000 feet is of Miocene age and is kn"own to have been up- 

 lifted at the end of the Miocene period, and if in the same 

 district there is an intermediate plain of early Pleistocene 



