OTHER UPPER CRETACEOUS ROCKS 



67 



Uppper Cretaceous rocks of the Santa Teresa Hills 



A sequence of sedimentary rocks of Late Cretaceous 

 age, consisting of massive gray medium-grained arko- 

 sic sandstone and tan or greenish-gray shales, is ex- 

 posed in the western part of the Santa Teresa Hills 

 and in thin fault silvers north of the Senator mine. 

 The sandstone is of special interest because it was used 

 in the construction of the buildings of Stanford Uni- 

 versity and also of several well-known public buildings 

 in San Jose and San Francisco. 



The sandstone of this unit is fairly well exposed, 

 even though it is deeply weathered as can be seen in 

 quarries more than 30 feet deep. The weathered rock 

 is generally found in clusters of large rounded boul- 

 ders scattered along lines of strike, but continuous 

 exposures measuring several hundred feet along the 

 strike are exposed in the vicinity of the quarries. 

 Where outcrops are sparse, the areas underlain by 

 sandstone can easily be distinguished from those un- 

 derlain by shale because they support a heavy growth 

 of brush. The bouldery outcrops show little bedding, 

 as the individual boulders commonly are derived from 

 a single bed. but in places the attitude of the bedding 

 is indicated by alinement of shale fragments or by thin 

 shale partings. The best exposures of the sandstone 

 are found in the vicinity of the westernmost rock 

 quarry in the Santa Teresa Hills shown in figure 50. 

 This quarry, having been cut into a dip slope of mas- 

 sive sandstone, is ideally situated to take advantage 

 of both the jointing and the bedding in the rock. The 

 individual beds as exposed in the quarry are as 

 much as 6 feet thick and are separated either by thin 

 shale partings or by layers containing abundant large 

 flakes of shale. Some bedding planes are marked by 

 thin layers containing small pebbles, most of which 

 are of dark chert, but the rock shows little tendency to 

 part along these pebbly layers. The shaly partings, 

 where exposed by stripping, exhibit many large worm 

 trails, and they commonly also show poorly formed 

 ripple marks. 



The characteristic, and locally spectacular, weather- 

 ing of the sandstone is best developed on the slope 

 above the quarry. Along this dip slope the least 

 weathered rock is cut by a rectilinear pattern of joints, 

 perpendicular to the bedding and spaced at intervals 

 of 8 feet or more. Upslope from the quarry the joints 

 have been widened by weathering, and the slope is 

 more and more deeply dissected toward the crest of 

 the hill. At the crest the edges of the dip-slope beds 

 are exposed and the joint pattern gives way to piles 

 of grotesque spheroidally weathered boulders. Some 

 of these boulders have a diameter of about 40 feet, 

 though most of them are somewhat smaller, and in 



FIGURE 50. One of the smaller quarries in the Santa Teresa Hills 

 from which Upper Cretaceous sandstone was taken for use as a build- 

 ing stone. The thick beds and barren dip slope combined to make 

 this an ideal site for a small quarry. 



several places they are precariously perched on the 

 dip slope of an underlying bed. Casehardening, due 

 to a concentration of limonite near the surface of the 

 rock, has formed on the rounded boulders crusts about 

 1 inch thick, which are generally cracked in a polyg- 

 onal pattern so that they resemble bread crusts, as is 

 shown in figure 51. Additional weathering along these 

 surficial cracks widens them until only small knobs 

 representing the centers of the polygons remain and 

 when these knobs have weathered away the process is 

 apparently repeated. Another type of weathering re- 

 sults in the formation of flat-bottomed caves. This 

 process can be followed from the development of small 

 flat basins with overhanging rims, resulting from the 

 enlargement, by standing water, of natural depressions 



FIGURE 51. Dome formed by spheroidal weathering in massive sand- 

 stone of the Upper Cretaceous rocks in the Santa Teresa Hills. The 

 casehardened surface and "bread crust" fractures are best developed 

 on these domes. 



