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<br /> <br />Or., ~ -:' <br />oj . .J . oJ <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />- <br /> <br />\ <br />~.-.., ",);k d'i ;-"J <br />,. ;;\~~.~ U <br />· '1J1~' ': .~~1lIftI <br />~ v-#".E; <br />. ~.7t~ *t_ .~: <br /> <br /> <br />Needle Mountains south of S;lverton. Colorado <br /> <br />leo .~oro <br /> <br />The oldest part of the San Juan River Basin. geologically. is in <br />the Needle Mountains area in Colorado where crystalline rocks of <br />Precambrian Age are exposed. These rocks consist of a complex <br />series of schists, gneisses. and granites. Rocks of Paleozoic Age <br />crop out on the southern and western flanks of the Needle Mountains <br />and in the adjacent Rico and La Plata Mountains in Colorado. These <br />racKs include quartzites. limestones. and shales in the lower part <br />overlain by red arkosic sandstones and conglomerates together with <br />beds of red muds tone and 5 il ts tone in the upper pa rt. I n the Monu- <br />ment upwarp area in Utah and Arizona, reddish sandstones and silt- <br />stones are exposed over extensive areas. Small outcrop areas of <br />sandstone Occur along the narrow canyons of the Canyon de Chelly <br />area in Arizona. <br /> <br />Mesozoic rocks crop out extensively in many parts of the basin and <br />consist of several thousand feet of alternating beds of sandstones, <br />siltstones. and shales with the sandstones predominating. Important <br />resources associated with these rocks include prominent coal beds <br /> <br />1lI-] <br /> <br />