Hekshey.] 



Quaternary of Southern California. 



There are those who will criticize even the above very 

 generalized correlation. They will maintain that the conditions 

 of erosion are so very different in separate areas and on different 

 formations that a comparison of valleys excavated has little 

 value.* So well aware of this am I that I usually purposely 

 avoid figures or the comparison of particular valleys in the 

 different areas. Erosion studies have not yet been reduced to 

 the precision of mathematics. The personal element enters very 

 largely into this department of geology. So many factors have 

 to be considered and given their true value, as the relative 

 resistance to erosion of different formations, the protection 

 afforded by vegetation, the slope of the surface, the annual 

 precipitation, and the distribution of the rainfall through the 

 year, that it is impracticable to subject the study to precise 

 rules. 



I have a mental picture of a great number of valleys in, say, 

 the Illinoian drift area of the Mississippi Basin. Some of these 

 are rock gorges, some are excavated in stiff boulder- clay and 

 some in loose sands. Some were eroded on steep hillsides and 

 some were carved from nearly flat plains. The whole taken 

 together give me an impression of the relative age of the 

 Illinoian drift sheet. As I travel over the Red Bluff area I 

 observe valleys excavated into solid and very resistant rock 

 beneath it, others in coarse gravel, and still others in loose, easily 

 eroded sand, as in the San Joaquin Valley. The whole system, 

 considered in connection with the conditions under which it was 

 eroded, gives me an impression of the approximate age of the Red 

 Bluff formation. When two impressions thus derived are similar, 

 I assume that the formations eroded do not very greatly differ in 

 time of uplift. Occasionally there will be encountered a valley 

 which seems to have been eroded under such definite conditions 

 as make it profitable to directly compare it with some particular 

 valley in the other area, thus giving the "impression" more 



*It is a matter of regret that no complete discussion of the erosion on the 

 various drift sheets of the Eastern States has yet been published. Perhaps the 

 most extensive references to the subject, so far, at least, as the older sheets are 

 concerned, may be found in Leverett's report on "The Illinois Glacial Lobe," 

 published as Monograph XXXVIII of the U. S. Geol. Survey. 



