GEOLOGIC MAPS AND NOTES ALONG HIGHWAY 49 BOWEN AND CRIPPEN 



47 





intra-niineral or post-mineral movement along the Mother Lode fault 

 system. 



The town of Coulterville is situated in a small valley which is at 

 about the same elevation as Bear Valley. Coulterville was founded about 

 1850 and was the center of a placer and gold quartz mining district which 

 embraced the Malvina, Louisa, Potosi, Champion, and Tyro mines as well 

 as the Mary Harrison. Many fine old buildings still remain including one 

 built and operated by Chinese. An old mine locomotive and other pieces 

 of early day mining equipment are on exhibit directly across the highway 

 from Division of Highways Historical Monument 332. 



COULTERVILLE TO CHINESE CAMP MAP 2 



Between Coulterville and Moccasin Creek canyon, Highway 49 

 passes over a rough region of serpentine and greenstone resembling the 

 terrain north of Bagby. The principal features of scenic and geologic 

 interest along this part of the route are the many white-tipped comb- 

 ridges which mark the surface trace of the Mother Lode quartz veins. 

 This series of quartz exposures was named the Penon Blanco or white 

 cliff by Spanish Californians. Although the long white walls are hardly 

 tall enough to be considered cliffs, the name has persisted through the 

 years. The weather-resistant quartz has kept the softer country rock, 

 which forms the flanks of the ridges, from being obliterated by erosion. 

 The best views of the Penon Blanco may be had at a sharp bend in the 

 highway 3.2 miles north of Coulterville, and again at the divide which 

 marks the boundary between Mariposa and Tuolumne Counties, about 

 five miles northwest of Coulterville. 



Two-tenths of a mile below the horseshoe bend where Highway 49 

 drops to the level of Moccasin Creek, the serpentines end against the 

 Mariposa slate. The latter lies in a deep synclinal trough which is dis- 

 rupted on both eastern and western sides by faults. The Mother Lode 

 thrust-fault system, which lies to the east of the syncline is a broad 

 mineralized zone which in places reaches a width of more than 180 feet. 

 Along the western fault contract, the Penon Blanco or Logtown Ridge 

 andesite agglomerate has been sheared into the Mariposa slate in many 

 places. 



At the Moccasin Creek dam and powerhouse of the city of San 

 Francisco, an unpaved road joins Highway 49 which connects with the 

 old placer mining district of Big Oak Flat. Big Oak Flat can also be 

 reached via Highway 120 which crosses Highway 49 2.2 miles farther 

 north. The placers of Big Oak Flat together with the adjoining Deer Flat 

 and Groveland districts are credited with production in excess of $25,- 

 000,000. A few miles northeast of Big Oak Flat in the vicinity of 

 Soulsbyville, several highly productive mines are located in wha't is 

 known as the East Belt. The East Belt roughly parallels the main Mother 



Lode about five miles to the east of it. Veins of this belt tend to be dis- 

 continuous and much narrower than the main Mother Lode system and 

 cut granodiorite or rocks close to it rather than meta-sediments and meta- 

 volcanics. The most productive mines in the Soulsbyville vicinity were the 

 Soulsby, with a production of $5,500,000, and the Black Oak with a 

 production of $3,500,000. 



A short distance beyond the intersection of Highways 49 and 120, 

 the former crosses the Tuolumne River and continues into Jacksonville 

 along the northwest bank of the river. This is the most scenic part of the 

 southern third of Highway 49. There are good picnic spots along the 

 river, several areas suitable for water sports, and the only eating places 

 between Coulterville and Jacksonville. 



Good exposures of greenstone and schists of the Calaveras formation 

 can be seen in road cuts at the south abutment of the Tuolumne River 

 bridge and westward along the roadbed of the Hetch-Hetchy Valley 

 Railroad to the serpentine contact. Calaveras rocks are also exposed in 

 roadcuts along Highway 49 from the bridge to the Harriman mine. The 

 latter is located close to the highway 1.8 miles southeast of Jacksonville, 

 and the headframe, sheet metal buildings, and mine dumps are very con- 

 spicuous at the roadside. Considerable capital was invested in this mine 

 but no great production was ever recorded. With the exception of the 

 Eagle-Shawmut, few mines in the Jacksonville district contained ore of 

 sufficient grade to warrant major development. The gold values are dis- 

 seminated through the wall rock in association with pyrite in what is 

 locally called gray ore. 



Less than half a mile southwest of Jacksonville and a short distance 

 west of the junction of Woods Creek with the Tuolumne River, the 

 abutments of old Moffats bridge can be seen jutting out into the Tuolumne 

 River. The vicinity of Moffats bridge and the adjacent Hetch-Hetchy 

 Valley Railroad cuts present good exposures of the Mariposa formation 

 in which fossils are found. These occur in a conglomerate and were first 

 described by H. W. Turner in the 1880 's. The fossils include cephalopod 

 and pelecypod molluscs of upper Jurassic age which correspond to 

 Oxfordian stage fossils of Europe. Fossil leaves are also present. West 

 from Moffats bridge, the Tuolumne River has cut directly across a 2500 

 foot ridge of agglomerate and serpentine. This represents a vertical 

 downcutting at the edge of the gorge of more than 1800 feet. 



Jacksonville, founded in 1848 and named for Colonel Alden Jack- 

 son, was a supply and amusement center for the mines along Moccasin 

 and Woods Creeks. There is little or no mining activity there now. A 

 paved road from Jacksonville connects with Jamestown via Stent and 

 Quartz. 



The extensive dumps and large mine buildings of the Eagle-Shaw.- 

 mut mine are located less than two miles northwest of Jacksonville at 

 the foot of the Shawmut grade. This is one of many major gold mines 



