78 



University of California Publications. [Geology 



STRUCTURE. 



FOLDING OP THE MERCED. 



It may be concluded that the folding of this series inaugu- 

 rated the present Coast Ranges, and probably took place not 

 earlier than the very end of the Pliocene. Lawson 's work on the 

 Peninsula of San Francisco shows that the Merced beds to the 

 very top at Seven Mile Beach are sharply tilted, while resting 

 unconformably upon these are strata of undoubted Pleistocene 

 age. This would seem to fix the uplift that disturbed the Merced 

 at the very close of Pliocene times. Recent work by Arnold* 

 points to the possibility of the upper portion of the Merced beds 

 at Seven Mile Beach being Pleistocene in age. If this is true, 

 the uplift did not take place until the Quaternary was some- 

 what advanced. In the present writer's field the St. Helena Rhy- 

 olite is folded with the sedimentaries and voleanics beneath it, 

 and the upper portion of these sediments contains shells of 

 Merced age. Furthermore, these rhyolites ("asperites") overlie 

 the Cache Lake beds of Becker, which Marsh called late Plio- 

 cene. This, again, puts the date of the uplift not earlier than 

 the end of the Pliocene. 



THE ST. HELENA FAULT. 



Northwest of Calistoga, on the western flank of Mt. St. 

 Helena is an important fault. The shape of the mountain when 

 viewed from the south suggests a fault block. The straight line 

 of the top sloping gently to the eastward, and the precipitous 

 descent to the west, have all the appearance of a tilted block 

 with a fault escarpment. A trip on foot from the summit down 

 the southwest side of the mountain, along the divide between 

 Knight 's Valley and the head of Napa Valley, demonstrated to 

 the writer a condition of things as illustrated in Section AB. 

 The throw of this fault must be at least 2,500 feet. 



Age. — This fault probably took place at the time of the uplift 

 which folded the Merced, since it appears to have been respon- 

 sible for the high ridge of which St. Helena and Cobb Mountain 

 are the culminating peaks, and to have preceded the great period 



*The Palaeontology and Stratigraphy of the Marine Pliocene and Pleis- 

 tocene of San Pedro, Cal., Memoir Cal. Acad, of Sciences, Vol. Ill, p. 13. 



