148 AKNOLD AND ANDERSON RECENT STREAM CONGLOMERATE 



that the stream in cutting it forms clean cross-sections througli all the 

 constituent materials. 



TOPOGEAPIIY AND GeOLOGY 



The topograph}' of the region is well shown in figure 1, plate 9. The 

 hills in the foreground are the remnants of a great synclinal valle)' plain 

 which once extended hetween the mountains in the distance and a similar 

 range back of the point from which the picture was taken. Evidences of 

 at least three terraces are preserved along the sides of the canyon which 

 winds down the old valley. It is on the middle one of these terraces, 

 which is usually 20 to 40 feet above the stream, and on the lower one 

 which forms the present valley floor and stream pavement, and which is 

 in most places a few feet above the channel of the stream, that the con- 

 glomerate occurs. 



The rocks of the main ridge north of the old valley just mentioned, 

 especially that portion in which White creek heads, belong to or accom- 

 pany the Franciscan formation, being largely sedimentary strata, ser- 

 pentine, glaucophane schist, and associated igneous and metamorphic 

 rocks of late Jurassic or early Cretaceous age; the rest of the contiguous 

 country, with the exception of a syncline of Pliocene sandstone under- 

 lying a portion of the old valley, is occupied by thick-bedded sandstone 

 carrying huge brown concretions, and thin-l^edded, dark clayey shale, 

 both of Cretaceous age. 



Description oe the Deposits 



White creek is a small stream and is intermittent in its lower course. 

 In the 9 miles between the serpentine area and its junction with Los 

 Gatos creek it descends 1,200 feet, with a fairly constant gradient of 

 from 2 to 21/0 per centum. The water is perfectly clear and, owing to 

 the white mineral deposit that it forms upon the pebbles and boulders 

 along its course, the water appears of a silvery white color. The white 

 deposit and cemented gravel is not apparent toward the head of the 

 stream, but is more and more in evidence farther down, along that part 

 of the course where (during the dry season) the water ordinarily reaches 

 the point of saturation with mineral matter and evaporates. In most 

 years the creek dries up or becomes very much reduced during the 

 summer months several miles above its confluence with Los Gatos creek. 

 The conglomerate extends for about 5 miles along White creek, begin- 

 ning at a point some 2 miles below the edge of the serpentine area, from 

 which the mineral matter of the cement is derived, and ceases about a 



I 



