24:-i 0. n. Hershey — River Terraces of California. 



required a long period of comparative stability during which 

 down-cutting in the basins was practically nothing and the 

 streams devoted their attention to widening their valleys. 

 The streams were low grade, at least in the basins, and had 

 meandering courses. Suddenly conditions changed and the 

 streams universally in this area began to trench the valley 

 floor, in many cases retaining the meandering courses. Several 

 hypotheses as to the nature of this change in conditions will 

 be examined briefly : 



1. Tiiat the down-cutting of the barriers in the gorges was 

 intermittent because of some peculiarity in their structure. 

 This position is untenable. The hypothesis would be worthy 

 of serious consideration if the strata were at a low angle and 

 hard and soft layei's alternated. The strata throughout the 

 region are practically vertical and the igneous rocks rise 

 vertically through them. Each barrier belt is essentially 

 uniform in structure and resistant properties in any given 500 

 feet of depth. All other conditions remaining equal, the 

 down-cutting in the gorges will be practically uniform and the 

 lowering of the basin floors equally as regular. The tiny 

 canons in the basins are rarely any wider than the streams. 

 Down-cutting in the gorges must proceed as rapidly as in 

 these canons in the basins. There is not nearly the contrast 

 in the size of the canons in the basins and the equivalent 

 portion of the gorges, as between the broad valleys and their 

 equivalent portion of the gorges. In this I see evidence that 

 the broad valley floors are not due solely to the barriers, but 

 to the barriers in combination with a past general low-grade 

 condition of the main streams of the entire area. Further it 

 appears evident that the down-cutting from the level of each 

 main terrace, and especially from the last, was due to causes 

 independent of the structure of the barriers. 



2. In some regions, particularly those of semi-arid climatic 

 conditions, an increased and better distributed rainfall some- 

 times causes a dissection of alluvial plains. In this region 

 precipitation is now, and apparently always has been abundant. 

 The streams have been able to remove the rock debris to the 

 sea about as fast as weathering produced it, so that it nowhere 

 accumulated to great depth. Therefore, widening of the 

 valleys occurred only under low-grade conditions of the main 

 streams. Suddenly increased rainfall would hardly result 

 in dissection, but rather, for a time at least, in aggradation. I 

 am going to connect the lower terrace with the later stages of 

 glaciation in the high mountains, and on the generally 

 accepted principle that the Glacial Period was one of excessive 

 precipitation, we must presume that the rainfall is now less 

 than when the broad valley floors were formed. 



