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Terra Nova, 23, 100-107, 2011 Abstract The Shannon Sandstone, one of the most controversial units in the field of stratigraphy, was first interpreted as a shelf sand-ridge deposit, but subsequent studies have furnished different interpretations, from lowstand-shoreface to incised-valley models. Despite a general consensus among previous workers about the significance of glaucony for Shannon interpretation, mineralogical characterization of the green grains has never been undertaken: identification of glaucony has been restricted to visual inspection. Accurate characterization of 'highly glauconitic' deposits from the Shannon Sandstone (Powder River Basin of Wyoming, Section W3) reveals that sedimentological interpretation of the greensands relies mostly upon mistaken mineral identity. Glaucony appears as a minor component of the rock and proves to be poorly mature and significantly less abundant (0-6%) than previously inferred (up to 85%). Characterization of the glauconitic minerals, however, provides data that contribute to Shannon interpretation. [ABSTRACT FROM AUTHOR]