76 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT. CALIFORNIA 



places outside the New Almaden district the forma- 

 tion is highly tilted, most of it within the district is 

 believed to be tilted no more than a few degrees from 

 its original attitude of deposition which, of course, 

 was nowhere quite horizontal. 



QUATERNARY ALLUVIUM 



Quaternary alluvium covers the floor of Santa Clara 

 Valley, and narrow tongues of it extend from the edge 

 of the valley for several miles up the larger tributary 

 canyons. The alluvium consists of silt, sand, and 

 gravel, and contains pebbles and boulders of rocks 

 evidently eroded from the nearby hills and moun- 

 tains. As it forms the principal storage reservoir for 

 the all-important ground water required for irriga- 

 tion in the Santa Clara Valley, it has been extensively 

 studied by W. O. Clark (1924). The reader desiring 

 more details as to the thickness of the alluvium, the 

 relative proportions of the various sizes of materials 

 it contains, or its porosity or permeability is referred 

 to Clark's report. 



Terrace gravels only slightly older than those on the 

 floor of Santa Clara Valley are perched on terraces 

 along the lower courses of the larger canyons as high 

 as 100 feet above the present canyon floor. These were 

 mapped in places along the Guadalupe and Los Gatos 

 Canyons, where they are fairly extensive, and similar 

 terrace gravels were also noted in Alamitos, Cherry, 

 and Llagas Canyons. 



LANDSLIDES 



More than 50 landslides are shown on the map of 

 the New Almaden district, and many more were noted 

 in the field but considered either too small or too 

 shallow to be worth mapping. Most of the larger 

 slides lie in a belt that strikes diagonally northwest- 

 ward across the district, from the southeast corner to 

 Blossom Hill. That so many landslides were mapped 

 in this belt is partly because the belt includes the 

 area that was mapped in greatest detail. But land- 

 slides are in fact especially numerous here, because 

 the rocks in this belt were originally less massive, and 

 are now more altered and sheared than those in other 

 parts of the district. 



The recognition of landslides is important in plan- 

 ning the location of adits, roads, or dams, because on 

 slides there is always danger of renewed movement. 

 Most of the slides in the district are easily recognized 

 by some physiographic expression, such as cirquelike 

 heads, hummocky surfaces, and distorted drainage; a 

 few of the older ones, however, have long been stag- 

 nant and can best be distinguished by their abnormal 



overlapping onto different kinds of rocks. If a slide 

 has descended a canyon, as many have in this district, 

 it is generally easy to recognize: where a canvon 

 crosses a slide it is more sharply incised than else- 

 where, and where a stream has passed around the toe 

 of a slide, its channel is obviously bent out of its nor- 

 mal course. 



The largest slides occupy nearly half a square mile, 

 and from these there are all gradations down to small 

 earth slumps. Their shapes are irregular, for many 

 of them are composites of several parts which have 

 slid at different times. The heads of the slides un- 

 commonly rounded and somewhat wider than their 

 lower parts, but many of the more irregular ones 

 have several heads and join downward to form a sin- 

 gle mass. 



Most of the larger slides are of the types classed MS 

 rockslides, consisting chiefly of bedrock and of debris- 

 slides (Sharpe, 1938, p. 74, 76) composed largely of 

 talus and soil. The distinction bet ween these two 

 types, however, must necessarily be rather arbitrary 

 in areas underlain by highly broken or crushed rocks 

 of the Franciscan group, because there is so little dif- 

 ference between the broken rock in place and the 

 debris derived from it. 



A good example of an earthflow. which moved 

 more slowly than landslides normally do, was mapped 

 topographically, on a scale of 200 feet to 1 inch, as a 

 part of detailed mapping of the surface above the 

 New Almaden mine. This flow is especially interest- 

 ing because it originated in 1927 and is therefore little 

 eroded; moreover, as the area had been accurately 

 mapped before 1927, on the same large scale, by the 

 mining company surveyors, the form of the surface 

 before and after the movement can he closely com- 

 pared. The change is illustrated by the profiles shown 

 in figure 55 which indicate the supposed position of 

 the sole of the slide. This sole is doubtless somewhat 

 more irregular than shown, but the angle of its slope 

 (14) cannot be far from correct. This flow, like 

 many slides in the area, shows many of the feature.- 

 generally associated with glaciers. Its head is cirque- 

 like, and a basin near the head contains a small pond. 

 The surface of the lower part is cut by many open 

 cracks like the crevasses of a glacier, and along the 

 lower margins in places there are low, linear mounds 

 resembling lateral moraines. The surfaces of slip- 

 page that are exposed show striae indicating the di- 

 rection of movement. 



Some of the small slides shown on the map of tin- 

 New Almaden mine area (pi. 3) originate in the large 

 mine dumps, but as the waste rock moves, it gener- 

 ally drags along the upper 5 to 10 feet of the under 



