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<br />~ <br /> <br />gentle and the eroded bedrock surface was strongly weathered. Gold- <br /> <br /> <br />bearing, quartz-rich gravels were deposited in the channels of the <br /> <br />Eocene rivers. The Eocene Yuba River had an extensive drainage area <br /> <br />which included the present area of the American River. <br /> <br />An outpouring of Miocene-age rhyolitic tuffaceous deposits choked <br /> <br />some streams of the northern portion of this ancient Sierra and buried <br /> <br />some land surfaces. Streams and rivers were forced to cut new chan- <br /> <br />nels. In Pliocene times, extensive outpourings of andesi tic flows, <br /> <br />tuffs, and DI1dflows covered much of the existing relief, thus forcing <br /> <br />streams and rivers into new channels. The American River drainage <br /> <br />was probably separated from the Yuba River drainage at this time or <br /> <br />right after the previous rhyolitic flows. Deepening of channels and <br /> <br />ri ver valleys ensued and much of the andesi tic tuffs and flows were <br /> <br />eroded away to be deposited in the Great Valley to the west. <br /> <br />At the close of the Tertiary'- about 3 million years ago - the pres- <br />ent Sierra Nevada came into being with an uplift of several thousand <br />feet and a subsequent downfaul ting to the eas t . Deep canyons were <br />cut, forming broad ridges capped with volcanic mud flows or tuff and <br />further exposing Wlderlying metalJK)rphic and granitic rocks. By this <br />time, JOOst of the Sierran Crest and the granitic rocks of the Crystal <br />Range were exposed and shaped. <br /> <br />5 <br /> <br />