EROSION IN ARIZONA BOLSON REGION 163 
underloaded, and therefore slowly cutting. This erosion would be 
increased merely by increasing the proportion of winter precipitation 
to the whole. : 
Some idea of the delicate balance between climate and geological 
processes is gathered, when one considers that a slight shifting of the 
precipitation from the summer to the winter seasons produces a 
marked effect in erosion. Furthermore, differences in rock structure 
are more reliable as records of past climate than erosional features, 
so full of double meaning. Further investigation into possible 
criteria of climate change will not be undertaken here, although all 
the criteria suggested by studies so far have not been exhausted. 
If I have suggested some of the possibilities of the final results of 
this study, some of the working methods to be applied, some of the 
points that call most urgently for investigation, and some of the 
results that may finally be expected, the purpose of this contribution 
will have been fully accomplished. 
