Based on a study of 10 drill cores, a sequence of oil shale and associated nahcolite, nearly 2,000 feet thick, in the lacustrine Green River Formation (Eocene) in the Piceance Creek basin, Rio Blanco County, Colo., was divided in ascending order into zones 1 to 13, B-groove, Mahogany zone (with lower, middle, and upper parts), and A-groove at the top. The odd-numbered zones and the Mahogany zone are mappable subsurface units of relatively thick oil shale and are distinguished from the even-numbered zones and A- and B-grooves which are thinner units of oil shale of lower grade. Large amounts of nahcolite found in zones 5 to 12 occur in (1) coarse-grained crystalline aggregates scattered through oil shale, (2) laterally continuous units of fine-grained crystals disseminated in oil shale, (3) brown microcrystalline beds, and (4) white coarse-grained beds that grade laterally into halitic rocks toward basin center. The original .upper limit of the nahcolite and halitic rocks is not yet completely known, but the present top is marked by a dissolution surface. Above this surface the rocks, extending from zones 11 or 12 upward into the Mahogany zone, form a water-saturated 'leached zone,' a geohydrologic unit in which large amounts of water-soluble minerals probably mostly nahcolite, and halite, were removed by ground-water dissolution. Rocks in the leached zone, mostly oil shale, are commonly broken and fractured and contain crystal cavities and solution breccias. Several solution breccias can be traced laterally into unleached beds of nahcolite and halite. Although evidence of salines is found in rocks above A-groove, the original saline facies that includes most of the bedded deposits extends from zone 5 upward into A-groove. Potentially ruinable beds of white nahcolite as much as 12 feet thick are found at depths of 1,560 or more feet below the surface. Some thicker beds of high-grade nahcolite are believed to be too close to the dissolution surface for safe room-and-pillar mining. Probably the most economical method of mining nahcolite would be as a coproduct of a shale-oil industry. Removal of nahcolite prior to retorting increases significantly the grade of oil shale by as much as 1.6 times, Several zones are more than 300 feet thick and average 30 or more weight percent nahcolite. Resources of nahcolite per square mile range as high as 489 million short tons. The total nahcolite resource in the basin is conservatively estimated at 32 billion short tons, which makes it the secondlargest deposit of sodium carbonate known .in the world.