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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />i I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Measured average annual snowfall varies from more than 460 inches at Ruby and <br />400 inches at Savage Basin, to approximately 170 inches at Crested Butte in <br />Gunnison County and 20 inches at Grand Junction. Upper basin lands have an <br />annual frost-free period of less than 70 days, while lower basin valleys near <br />Grand Junction average around 190 days. <br /> <br />2.4 GEOLOGY <br /> <br />The bedrock of the Gunnison Basin can generally be divided into three <br />rock types; (1) crysta 11 i ne i ntrusi ve and metamorphi crocks, (2) sedimentary <br />rocks, and (3) rocks of volcanic origin. The oldest rocks in the basin are <br />1.4 to 1.8 billion years old (Precambrian). They are crystalline intrusive or <br />metamorphic rocks such as granite, gneiss, or schist. The rocks in the Black <br />Canyon of the Gunni son are typi ca 1 of Precambri an crysta 11 i ne rocks in the <br />region. <br /> <br />The sedimentary rocks of the basin were deposited during two eras of <br />geologic history. The older sedimentary units include the leadville Limestone <br />which was deposited in a shallow ocean during the early Paleozoic Era (320 to <br />570 million years ago). The Maroon Formation, consisting of arkosic sandstone <br />and conglomerate, was deposited alongside the ancestral Rocky Mountains which <br />were uplifted about 320 million years ago. <br />The second era of sedimentary rock deposition was the Mesozoic, when the <br />mountains had been eroded and the region was located at an elevation at or <br />below sea level. These younger sedimentary rocks include thick sequences of <br />sandstone, shale, coal, and some conglomerates. These rock units include the <br />Mesaverde Format i on, Dakota Sandstone, Mancos Shale and Morri son Format i on. <br />These rocks cover the western portion of the basin. <br /> <br />The structure of the rocks in the basin varied dramatically from east to <br />west. In the eastern part of the basin the Precambrian and Paleozoic rocks <br />have been uplifted and exposed by large normal faults and minor thrust faults. <br />The sedimentary rocks, including the younger rocks, have been folded and <br />faulted. In the western part of the basin the sedimentary rocks are <br /> <br />" " <br />! 1- <br />',,0 __ <br /> <br />2-3 <br /> <br />I il0003151 <br />