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University of California . 



I Vol. 3. 



loess is slightly more resistant to erosion than the loose sand 

 of these lower San Pedro terraces. Also, it is probable that 

 in prehistoric times, the San Pedro terraces were less protected 

 by vegetation than was the loess terrace. This is made up to 

 approximate equality by the rainfall of the San Pedro region 

 being less than that of the Illinois-Iowa region. The San Pedro 

 terraces are on the border of the sea, in the rainiest part of 

 Southern California, so that the conditions are not by any 

 means those of the arid region. The average annual precipita- 

 tion at the neighboring city of Los Angeles is about 17 inches, 

 and that of the Illinois-Iowa area about 33 inches. It is 

 believed that the greater rainfall of the latter region verv nearly 

 balances the less resistant nature and poorer protection of the 

 sand of the lower San Pedro terraces. 



The amount of erosion accomplished in the two areas is the 

 same. Both systems of terraces are dissected by small canon- 

 shaped valleys and ravines and the same amount of the surface 

 retains its original flatness. I was very strongly impressed by 

 the evidence that the lower terraces are approximately Iowan in 

 age. Certainly they cannot be much older. 



The upper terraces are on a steeper slope, are narrower, con- 

 sist mostly of benches cut into the Miocene rocks, and show 

 greater erosion than the lower three. Loose fragments of the 

 soft sandy shales of the Miocene, showing the peculiar and char- 

 acteristic marine abrasion, are scattered on each terrace includ- 

 ing the summit of the hill where they are in sufficient abundance 

 to remove any doubt of the submergence of the very highest 

 point. The old sea-cliffs can be traced and several of the 

 originally best developed terraces are yet quite distinct. Ravines 

 of considerable size have been excavated in the soft rock of the 

 steep slopes of the hill and these upper terraces are not by far as 

 well preserved as the lower three even though they consist of a 

 much more resistant material. The evidence of their age is less 

 conclusive than in the case of the first discussed members of the 

 series, but I am pretty well satisfied that the higher terraces are 

 considerably older than the last three. Apparently the elevation 

 began in the Quaternary era at a time long preceding the Iowan 

 epoch and continued intermittently until that epoch. 



