EROSION AL HISTORY OF DRIFTLESS AREA 49 



age at an average altitude of 1,000 feet, the conclusion is 

 warranted that the land was degraded 1,000 feet during the 

 Pliocene period. If it be assumed that all this degradation 

 took place at a rate which is average for all lands through 

 all times and that this average rate is 1 foot in 9,000 years, 

 the duration of the Pliocene period would be estimated at 

 9,000,000 years. The estimate, of course, would be subject 

 to large error in each of the two points of the assumption. 

 However, this method of estimating the duration of geologic 

 time, duly considered and qualified, might be as accurate 

 as estimates based on the rate of accumulation of sediments. 

 the rate of increase of salinity in the sea, the rate of life 

 evolution, or the rate of radio-active changes. 



CONCLUSION 



Fi'om the foregoing discussions it seems clear that there 

 are many rules for determination of the various events in 

 the e}'o&ional histories of regions, that all of them are open 

 to exception and some of them to serious and frequent ex- 

 ceptions, that the full interpretation of erosional history is 

 attended with great diftlculty, that such interpretation is 

 safe oiily after wide areas have been studied closely, and 

 with all criteria and limitations in mind, but that on the 

 whole fairly accurate conclusions may be drawn by the :^tu- 

 dcnr of diligence, persistence and analytic mind. 



