616 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



While standing at high angles, these beds do not show any important faulting along either 

 of the sections. From near Knoxville, where they appear to overlie a large laccolith of serpen- 

 tine, they extend in unbroken succession with steep northeasterly dip, to the head of Capay 

 Valley, at Rumsey, where they are covered by Tertiary gravels and sandstones. The average 

 angle of dip from Knoxville to Rumsey can not be less than 45°, which would give the series a 

 thickness of 4 miles, and this does not represent the whole of the accumulation of sedimentary 

 beds, since the upper limit is not exposed. 



In the Coast Range south of San Francisco the Lower Cretaceous strata are 

 less fully represented than they are to the north. In the Santa Cruz quadrangle ^'' 

 the EJioxville is but 100 feet thick and is bounded by unconformities above and 

 below. (See Chapter XV, p. 669.) In the San Luis quadrangle ^^'* the relations 

 are as follows: 



Strata of Cretaceous age in California include three main groups — the Eaioxville, Horsetown, 

 and Chico. Two of these, the Knoxville and the Chico, are widely distributed throughout the 

 Coast Ranges, and their representatives, the Toro and Atascadero formations, are found within 

 this quadrangle. Nothing corresponding to the Horsetown group has been recognized here, 

 and probably the horizon is represented by an erosion unconformity. 



It has been thought that the Cretaceous sediments in California formed a conformable 

 series from top to bottom, but, in the central Coast Ranges at least, this period was broken 

 two or more times by igneous outbursts, with one long interval of elevation aad erosion. 



The Toro formation within the San Luis quadrangle consists of more than 3,000 feet of dark 

 shale and thin-bedded sandstone. The formation is named from a creek which flows across it. 

 The shale forms almost the whole of the bottom and middle portions. The sandstone is more 

 abundant toward the top. The formation is not supplied with many fossils, for, excepting 

 one specimen of an ammonite, the only species found is one belonging to the genus Aucella. 

 This is very abundant through the middle and lower portions of the formation. 



The Toro formation is the local representative of the Knoxville group, but it probably 

 corresponds to a small part only of Kjioxville time, the rest being represented by tht? uncon- 

 formities above and below. It belongs to the lower Cretaceous or Shasta series. 



The Cretaceous sequence on the west side of the Valley of California, near its 

 south end, in the Coalinga district, is thus described by Arnold and Anderson: ^^^ 



The next to the oldest terrane exposed in the Coalinga district is a thick series of strata 

 of sandstone, shale, and conglomerate overlying the Franciscan formation and covering a wide 

 belt for the most part west of the foothill region. It forms the high ridges bordering the Coalinga 

 district on the west and may be easily recognized by the dark, thinly bedded, compact shale 

 and sandstone of its lower portion and the massive drab concretionary sandstone of its upper 

 portion. These strata are of Cretaceous age and comprise part or all of the two formations 

 well known elsewhere on the west coast as Knoxville (Lower Cretaceous) and Chico (Upper 

 Cretaceous). * * * 



The Horsetown formation, which forms the middle portion of the Cretaceous in the standard 

 Coast Range section, is not known to be represented in the Coalinga district, although it is not 

 impossible that a portion of the great thiclcness of nonfossiliferous Cretaceous strata may be 

 the equivalent of this formation. No evidence, however, has yet been found of its presence 

 in this portion of the Coast Ranges. 



A conservative estimate places the total thickness of the strata mapped as KnoxviUe- 

 Chico at 12,800 feet. The maximum thickness is probably much greater. This succession of 

 strata may be divided hthologicaUy into three main diAdsions. The total thickness of the middle 

 and upper divisions, which are probably to be referred together to the Chico formation, is at 

 least 9,500 feet. A thickness of 8,300 feet is measurable in single sections, and even greater 

 thicknesses may be found in single sections of the Kjioxville-Chico as a whole. It is hkely that 



