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University of California. 



[Vol. i. 



or genetic unity will probably not be questioned, the name which 

 appears at the head of the paragraph is proposed. Its total length 

 from the Golden Gate southwestvvard may be placed at 150 miles. 

 The breadth of its bottom varies from that of a head-water gorge 

 to about 17 miles at the Bay of San Francisco. It has a remark- 

 ably straight course for its entire length. This valley is occupied 

 by a trenched and terraced Pliocene delta. This fact is not so 

 apparent at the middle or Santa Clara portion of the valley as it 

 is at its lower and upper portions. The lower portion is partially 

 occupied by the Merced series, which has already been described. 

 The upper portion, from the Pajaro Canon southeastward, reveals 

 the delta in a great volume of appromimately horizontal gravels. 

 The Pliocene age* is established by their unconformable relation 

 to the upturned Miocene of Pajaro Canon, and by the fact that they 

 contain pebbles of the white, siliceous shale which characterizes the 

 Miocene of the coast. These gravels are exposed on the San Benito, 

 the Tres Pinos, and Los Meritos in a series of very remarkable 

 cliffs, often over 1,000 feet high, which are being developed by a 

 vigorous sort of sculpture which yields the effects of a "bad land" 

 topography (see Plate 9). The trenching and terracing action of 

 the streams, as they have by stages dissected the delta during the 

 progress of the uplift, has left remnants of it in the form of isolated 

 hills and plateaux in the middle of the valley. One of the highest 

 of these lies just above the confluence of the Tres Pinos and the 

 San Benito between the two streams. This plateau shows mag- 

 nificent cliff sections, particularly on the San Benito side, and the 

 character of the ridge as a series of well-bedded gravels from top 

 to bottom is evident to the most casual glance. The bedding is 

 either horizontal or is tilted to the eastward at angles up to perhaps 

 15 . The altitude of the summit of the gravel plateau was made 

 the subject of careful measurement. By the use of the mercurial 

 barometer the summit was found to be 929 feet above Tres Pinos 



* Whitney recognized these gravels as Pliocene, but without, as Dall points 

 out, adducing evidence for this conclusion. In his "Geology of California" 

 he gives them but a passing reference, without expressing an opinion as to 

 their age. In his later work, "The Auriferous Gravels," without having fur- 

 ther examined the field, he simply states that they are of Pliocene age. 



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