CHANGE-PRODUCING AGENCIES 



657 



The cutting down of the valleys, observed there would represent 

 periods of lesser precipitation, and the refilling would mark periods of 

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Figure 6. — Tentative Correlation of Human and Physical Evente during Quaternary and Recent Time. 



The information obtained in the shafts is brought together in figure 5, 

 in which the essential facts are represented correctly in the vertical scale, 

 while in the positions of the valley walls are necessarily idealized. 



