1912] 



Clark: Neocene Section at KirJcer Pass 



53 



Lower Division. — That part of the San Pablo series which 

 is here designated as the Lower Division has a thickness, as 

 measured to the west of Kirker Creek, of about 350 feet. The 

 basal beds of the Lower Division in this vicinity are composed 

 of a layer of coarse conglomerate which is in places ten to 

 fifteen feet thick. The conglomerate consists of well-rounded 

 pebbles, some of which are three to four inches in diameter. 

 They are mostly made up of quartzites and volcanics, together 

 with some boulders of sandstone and shale. In the vicinity of 

 Markley Canon the basal beds consist of a layer of massive to 

 very much cross-bedded, coarse to conglomeratic sandstone, that 

 stands out distinctly from the gray sandstones above, because of 

 its reddish-yellow color. 



That the San Pablo series in this region rests unconformably 

 on the Monterey is shown by the following facts: (1) There is a 

 difference in strike and dip, and a slight irregularity in contact 

 between the two. (2) The basal beds of the series appear to rest 

 upon different members of the Monterey as one follows the line of 

 the strike. (3) Borers of the pholadid type were found along 

 this contact for a distance of over three miles. (4) Well-rounded 

 boulders lithologically identical with the tuffaceous sandstones 

 and the shales of the Monterey below, are found in the basal con- 

 glomerate. These facts, together with the sharp lithological 

 break, leave no doubt as to the unconformable relationship of the 

 San Pablo and the Monterey. 



In the vicinity of Kirker Creek the strike and dip of the 

 Monterey and the lower beds of the San Pablo are practically 

 the same, but to the east of Markley Canon there is a difference 

 in strike of from 10° to 15° and at least 5° difference in dip. 

 As pointed out above, the tuffaceous beds of the Upper Monterey 

 gradually wedge out in going east from Kirker Creek and disap- 

 pear at Markley Canon, the basal beds of the San Pablo resting 

 on successively lower horizons of the Monterey. 



The Lower Division of the San Pablo in general may be 

 characterized as made up of coarse, massive sandstones, con- 

 glomerates, and a very minor amount of finer material. Cross- 

 bedding is very evident at different horizons, and in some locali- 

 ties, especially to the west of Kirker Creek, considerable tuff 



