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POPULAR SCIENCE MONTHLY 



Francisco southward along the coast for -i miles, beautifully exposed 

 in bare cliffs 700 feet high. A study of the geology shows that 

 this fragment lies between two fault planes, beyond which no trace 

 of it is found for many miles. Calculation shows that on either 

 side of this depressed block there has been elevation during Quater- 

 nary time of about 1% miles with pari passu removal of the Merced 

 series and deep erosion of the underlying basement. The time interval 

 is best measured by the erosion of the mile of Merced sediments. As 

 exposed to-day upturned along Seven-mile Beach, these consist mainly 



FRANCISCO 



Pescadero Pt 



Fig. 1. Sketch Map of Northern Part of San Francisco Peninsula, showing position 

 of the three faults to which the discussion in this paper is confined. 



of half consolidated sands, clays and hard conglomerates, ringing under 

 the hammer. To have removed such a volume of rock would appear 

 to have required most of Quaternary time, especially as the underlying 

 hard pre-Tertiary sandstones, serpentines and cherts were also deeply 

 eroded. During this movement there were volcanic outpourings of 

 some extent. There also appear to have been interruptions to the 

 movement which do not affect the main problem now, and will be 

 passed by. The final result probably left the whole region about San 

 Francisco dry land, except for the water courses. Sand dunes and 

 wash deposits abounded then as to-day. The Monterey cypress and 

 Douglass spruce grew abundantly near San Francisco, some of the 

 latter reaching a diameter of four feet, or more. The mammoth or 

 mastodon roamed the peninsula at that time. 



