oded ▁enorm ous ▁amounts ▁of ▁rock ▁and ▁sed iment ▁from ▁the ▁mountains . ▁Most ▁of ▁the ▁S ier ras ▁are ▁under lain ▁by ▁gran it ic ▁ igne ous ▁and ▁met am orph ic ▁rock ▁d ating ▁back ▁to ▁the ▁Mes oz o ic ▁Era ▁( 2 5 0 - 6 6 ▁MY A ); ▁in ▁addition ▁many ▁of ▁the ▁San ▁Jo aqu in ' s ▁trib ut aries ▁flow ▁across ▁a ▁fo oth ills ▁region ▁of ▁met am orph osed ▁vol can ic ▁rock ▁more ▁fam ously ▁known ▁as ▁the ▁Mother ▁L ode ▁Gold ▁B elt . ▁ ▁The ▁ ▁lower ▁part ▁of ▁the ▁river , ▁in ▁sharp ▁contrast , ▁is ▁a ▁me and ering ▁stream ▁flow ing ▁over ▁C en oz o ic ▁all uv ial ▁depos its ▁( 6 6 ▁MY A - present ), ▁which ▁together ▁compr ise ▁the ▁flat ▁floor ▁of ▁the ▁Central ▁Valley . ▁The ▁trem end ous ▁volume ▁of ▁sed iments ▁that ▁under lie ▁the ▁lower ▁San ▁Jo aqu in ▁River ▁ranges ▁from ▁ ▁deep , ▁with ▁distance ▁to ▁bed rock ▁generally ▁increasing ▁in ▁a ▁nor ther ly ▁direction . ▁Prior ▁to ▁the ▁u pl ift ▁of ▁the ▁California ▁Coast ▁R anges , ▁more ▁than ▁ ▁of ▁sed iments ▁were ▁depos ited ▁at ▁the ▁foot ▁of ▁the ▁S ier ras ▁by ▁tid al ▁activity , ▁and ▁the ▁ancest ral ▁San ▁Jo aqu in ▁and ▁its ▁trib ut aries ▁flow ed ▁west ▁over ▁this ▁all uv ial ▁plain ▁to ▁the ▁sea , ▁dump ing ▁their ▁own ▁sed