GEOLOGIC HISTORY OF THE SIERRAN GOLD BELT JENKINS 



27 



batliolithic granites form what is commonly known as the "Bedrock 

 series" of the Sierra Nevada and all of the gold-bearing quartz veins 

 are enclosed in these hard rocks of the early geologic periods. 



The only definite rock record of Cretaceous (which followed the 

 Jurassic) in the Sierra Nevada is a series of sandstones and shales (not 

 metamorphosed) of Upper Cretaceous age that lie well down on the west- 

 ern flanks and on the deeply eroded surface of the Bedrock series. This 

 means that there is a break in geologic evidence representing the unre- 

 corded time of the early Cretaceous period ; a break caused by a long ero- 

 sion period. Even the tops of the quartz veins were reached. The rocks of 

 this early mountainous surface were torn down and rolled into boulders 

 and sands and deposited in a Cretaceous sea that spread out and filled a 

 basin where the Great Valley and Coast Ranges now are located. The 

 mountain range of the ancestral Sierra even extended as far as the center 

 of the present Great Valley during early Cretaceous, but erosional 

 removal of the mountains ' western front had the surface effect of moving 

 the range eastward. 



This brings the geologic history to another most interesting epoch of 

 events which have had a profound effect upon the entire civilization 

 of California. 



The erosion period of the Cretaceous laid bare the upper, richer 

 parts of the gold-bearing quartz veins, and the episode which followed 

 resulted in releasing, removing, depositing and concentrating the gold 

 from the upper portions of the veins into placer deposits of unbelievable 

 richness. The gold-releasing episode was a period of geologic quiet, not 

 mountain-making nor destructive erosion. It occurred during the Eocene 

 period and particularly during its beginning or Paleocene time when the 

 climate was semi-tropical and humid. The effect of this environment on 

 a land surface which had already been eroded partly to base level and 

 partly to rolling hills, bordering a lagoon-margined sea, was to cause 

 deep decay and weathering resulting in the formation of deep red lateritic 

 soil such as one finds in the tropics. The rocks were so badly decayed that 

 clay was formed and floated into the lagoons where it was deposited along 

 with decayed vegetation to form lignite. Quartz particles did not decay 

 with the feldspar but remained as quartz fragments, pebbles, and quartz 

 sand. In places they were cemented together by iron oxide, weathered 

 from the rock. The gold in the veins was thus separated from its matrix 

 and released. It then found its way, because of its high specific gravity, to 

 the lower parts of crevices and into the beds of swift streams, where it was 

 caught in the natural rock-riffles of slate and schist. These Eocene 

 streams, however, were streams of 50 million years ago and they were 

 destined to undergo entrapment and to be preserved until a much later 

 time. 



The entrapment of streams, or rather of the boulders, gravels, and 

 sand deposited in their channels and along their banks, was effected in the 

 following manner : A fall of finely divided, light-colored, volcanic ash 



(rhyolitic in composition) initiated a period of voleanism as the epoch of 

 Eocene time closed. This period of voleanism continued in Oligocene and 

 Miocene time. The volcanic ash was washed into the streams where it 

 dammed them, formed lakes, covered them, and diverted the drainage. 

 The voleanism did not wane, but increased in vigor. It was accompanied 

 by earth movements and severe earthquakes. The low hills of the area 

 started to rise with the result that the flow of the newly diverted streams 

 was accelerated. Darker colored lavas (andesite), mud flows, volcanic tuff 

 and ash spread far and wide, filling all depressions, and finally leaving 

 only the higher mountains and peaks standing as islands above this steam- 

 ing sea of volcanic mud. 



This extreme activity of voleanism was extended far to the north, 

 even to the Columbia lava plateau of eastern Oregon and Washington, 

 and there were outbursts in many other parts of California and the West 

 in general. All these events took place largely in the Miocene and Pliocene 

 epochs of the Tertiary. 



Finally toward the close of the Pliocene, as this period of voleanism 

 came to an end, a new epoch of disturbance came into being. Volcanoes 

 broke out at the Dardanelles and elsewhere high on the Sierra, and one 

 molten flow of basic lava followed the canyon of a stream cut largely in 

 ash as far as Knights Ferry. Later erosion of the softer ash left the hard 

 black lava stream, sinuous as a river in form, high and dry as we see 

 Tuolumne Table Mountain today. 



Earthquakes must have been of tremendous intensity, resulting from 

 great earth movements and faulting which prevailed during late Pliocene 

 and early Pleistocene. The Sierra Nevada province moved largely as a 

 unit, tilting westward and breaking along the eastern escarpment. Accu- 

 mulated fault displacements on this eastern front amounted to many 

 thousands of feet. The streams were accelerated manyfokl, and torrents 

 swept down the western slope of the Sierra, cutting deep and rugged 

 canyons. The high Sierras became enveloped and packed in snow; then 

 glaciers crept down from cirques, their places of origin near the highest 

 peaks. 



In many places, especially in the lower slopes of the Sierra Nevada 

 where the foothills begin to merge into the Great Valley, the volcanic 

 cover of soft ash has been largely stripped by erosion from the harder 

 surface of bedrock. Thus were exhumed parts of the old Eocene surface, 

 which was formed 50 million years ago, with its weathered mantle and 

 deep red soil, its cover of white quartz gravel, and its rich concentrations 

 of gold lying in the beds of thousands of ancient stream channels. 



This old rolling surface now remains between the newly formed can- 

 yons. To the east it rises gradually in elevation while the canyons increase 

 in depth. It is easily understandable why these Eocene channels, 

 originally controlled in direction by bedrock structure over which the 

 water flowed, have no parallelism to the new streams born on the west- 

 ward sloping surface of the widespread volcanic cover, and destined later 



