San Francisco Bay and Golden Gate 149 



that flow down the northeast slope of the faulted block are 

 "subsequent" streams. Their courses have been determined by 

 rock structure rather than by normal slope. Drainage is 

 toward Suisun Bay. The streams follow lines of faulting 

 rather than that of the general slope of the land surface. The 

 Berkeley block is shattered by many faults. These breaks in 

 the rocks offer easy paths for the waters which flow from the 

 crest of the block northeastward. Since the courses of the 

 streams have been determined by rock structures, breaks or 

 weak places in the rocks, after or subsequent to the uplifting 

 of the great block, they are called subsequent streams. 



Alameda Creek enters San Francisco Bay near its southern 

 end. Its headwaters come from the slopes of Mt. Hamilton on 

 the south and Mt. Diablo on the north, and drain broad low- 

 lying valleys east of the Berkeley Hills. It crosses the hilly 

 country of the east slope of the Berkeley uplifted block. The 

 course of the stream across these hills was evidently established 

 before the uplifting of the block, and the stream has continued 

 across the rising mountain block cutting its rugged canyon 

 deeper and deeper into the hard rocks as the block was uplifted. 

 It is an example of an "antecedent" stream, because its course 

 had been established before the uplifting of the block, that is, 

 antecedent to the uplifting, and has held its course while the 

 uplifting of the block was in progress. An antecedent stream 

 is one whose course was established before an obstacle arose 

 across its path and has persisted in its course despite the obstacle. 



Due to changes in elevation that affected the region south 

 of Mount Diablo the waters from Livermore Valley, which for- 

 merly had reached Suisun Bay through the broad San Ramon 

 and Ygnacio valleys, were diverted to Alameda Creek. The 

 uplifting of the land between the San Ramon and Ygnacio val- 

 leys on the north and the drainage basin of Alameda Creek on 

 the south enabled the tributaries of Alameda Creek to push their 

 heads back and tap the streams that had flowed north. The 

 divide between the head of San Ramon Creek and the drainage 



