The geology of Utah includes rocks formed at the edge of the proto-North American continent during the Precambrian. A shallow marine sedimentary environment covered the region for much of the Paleozoic and Mesozoic, followed by dryland conditions, volcanism and the formation of the basin and range terrain in the Cenozoic. Utah is a state in the western United States. Geologic History, Stratigraphy & Tectonics The eastern Uinta Mountains near the Colorado line and the Raft River-Dover Creek Mountains contain the oldest rocks in Utah from more than two billion years ago. Rubidium-strontium dating of the Red Creek Quartzite in 1965 indicated an age of 2.3 billion years. Other geologists found 2.4 billion year old schist and gneiss in the Albion Range Green Creek Complex. Prior to the 1960s, geologists inferred the Vishnu and Farmington Canyon gneiss and schist as Archean in age, but subsequent research indicated an formation between 1.6 and 1.5 billion years ago in the Proterozoic. From 1.75 to 1.65 billion years ago, Utah was impacted by the Matzatzal Orogeny. Thick sequences of sedimentary rocks from the Neoproterozoic, including tillite underlie much of the state. Paleozoic (541-251 million years ago) Utah has numerous rocks deposited during the Paleozoic, as multicellular life diversified for the first time. Basal Cambrian quartzite, up to thick, is sometimes difficult to distinguish from Precambrian rocks. A shallow marine environment produced mottled limestone and stromatalite beds, deepening to the west. Thin shale and siltstone units of the Whirlwind Formation likely originated from sediment sources in the east. The Pioche Shale contains some of the oldest Cambrian fossils, followed by a continuous sequence of trilobite fossils in the House Range. In eastern Utah, which rests on the craton of the proto-North American continent Laurentia, there are no Ordovician deposits, but Early Ordovician clastic limestone, Middle Ordovician quartz sandstone and Late Ordovician dolomite is common in the west. Geologists have found intraformational limestone from the Early Ordovician, formed as limestone forming in a tidal bed is redeposited as pebbles in situ. Brachiopod, trilobite and echinoderm fossils are common in these rocks. Middle Ordovician quartzites form distinctive orange, brown and pink cliffs above gray limestone slopes, set beneath black dolomite cliffs in the north and west. Chert rich dolomite cliffs form in the most widespread Ordovician unit: the Fish Haven-Ely Springs Dolomite. Only a single stratigraphic unit—the Laketown Dolomite—formed during the Silurian, far from sediment sources in