Lawson ~i 



I»A1 Al in J 



The Berkeley Hills. 



Botli limestones are distinctly stratified and are locally replaced 

 by chert. They are compact, light gray rocks of uniform texture, 

 occasionally containing fresh-water fossils, such as Planorbis and 

 Physa. Locally they contain numerous detrital fragments, indi- 

 cating a shallow-water origin for the deposit. They can scarcely lie 

 regarded as other than chemically-precipitated deposits, although 

 the conditions which determined the precipitation can not be stated 

 positively. 



In addition to the sandstones which were found on the south- 

 western limb, and which are also here represented, there are thick- 

 beds of pebbly conglomerates. These are found in immediate 

 stratigraphic association with the limestones. Besides these, there 

 are also beds of volcanic tuff, which, although of no great thickness, 

 are a persistent feature of the northeastern limb of the syncline. 

 Our former statement as to the character of the beds which make 

 up the Siestan formation is thus considerably enlarged by an 

 examination of these outcrops. 



Northeastern Limb of Siestan Syncline. — If we follow, now, the 

 outcrop of the northeastern limb of Siestan syncline to the north- 

 west, we find, as soon as we climb out of Siesta Valley, that the 

 zone of outcrops is along a shelf or terrace between two escarp- 

 ments, the lesser above and the greater below, just as it is on the 

 front of Frowning Ridge. This terrace is a prominent feature of 

 the northeast side of San Pablo Ridge, and is in general much 

 broader than the similar feature on Frowning Ridge, owing to the 

 greater volume of strata on this limb of the syncline. As we 

 follow the terrace, the exposures show that the limestones, clays, 

 and tuffs are persistent, but sandstones and conglomerates are not 

 in evidence. On reaching Bald Peak the outcrop bears more to the 

 west, and the terrace passes into the bottom of a valley, a fork of 

 Wildcat Canon. 



The limestones and cherts are traceable for about half a mile 

 down this depression, and beyond that only the clays and tuffs are 

 found to persist. The formation as a whole does not, however, 

 diminish in volume in this direction as it does on the other limb of 

 the syncline. Towards the outlet of Baffling Gulch extensive land- 

 slides have occurred in the soft clays just as they have in Siesta 



