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B-3 <br />extend into modern river channels and flood plains. Overlies buried older gravel along <br />the Arkansas River. Thickness about 10 ft. Areas mapped as alluvium may be prone to <br />flooding and sediment deposition. The unit is typically a good source of sand and gravel. <br />Qat Younger terrace alluvium (Holocene) -Consists of poorly sorted, clast-supported, <br />cobble, pebble, and locally boulder gravel in a silty, sandy matrix underlying ten•aces 10 <br />to 15 ft above modern stream channels. Fine-grained overbank deposits may be present <br />locally. Deposits underlie alluvial terraces in Trout Creek and its larger tributaries. <br />Clasts aze generally unweathered and abundant on the surface. Soil development on <br />terrace surfaces is weak. Generally post-dates the Pinedale glaciation, so is considered <br />early to middle Holocene in age. Maximum exposed thickness is 15 ft. The unit may be a <br />good source of sand and gravel. <br />Qpoy Pinedale outwash deposits, younger (latest Pleistocene) -Yellowish-gray, crudely <br />stratified alluvium containing well-rounded to subrounded boulders, cobbles, pebbles, <br />and sand. Composed principally of Tertiary igneous rocks and Precambrian metamorphic <br />and igneous rocks. Soil at top is weakly developed. Forms a terrace about 30 ft above <br />Arkansas River. Upper parts of outwash contain huge boulders emplaced by two <br />catastrophic floods from breakouts of glacier dams at Pine Creek north of the map area <br />(Scott, 1975). Average size of boulders is about 4 to 6 ft, but the largest boulder <br />recognized is 65 ft in diameter. In places, Pinedale outwash can be subdivided into three <br />terraces, from oldest to youngest, Qpoyl, Qpoy2, and Qpoy3. The unit is a commercial <br />source of gravel. Thickness probably 10-30 ft. <br />Qpoo Pinedale outwash deposits, older (late Pleistocene) -Yellowish-gray crudely stratified <br />alluvium containing well-rounded to subrounded boulders, cobbles, pebbles, and sand. <br />Composed principally of Tertiary igneous rocks and Precambrian metamorphic and <br />igneous rocks. Soil at top is weakly developed. Forms most of the valley floor in the <br />vicinity of the Arkansas River, in a terrace about 65 ft above the river. Upper parts of <br />outwash contain huge boulders emplaced by two catastrophic floods from breakouts of <br />Buena V ista East quadrangle <br />