Illite and poorly crystalline chlorite are the dominant clay minerals in the Wolsey Shale, a transitional unit between the Flathead Sandstone below and the carbonate Meagher Formation above. The clay minerals were deposited as partly degraded, land-derived muds. Glauconite pellets are allochthonous in origin and are composed of lM dioctahedral illite. Carbonate facies range from fine to coarse grained and reflect sedimentation close to slightly uplifted source lands and involving sporadic wave attack on semiconsolidated, highly argillaceous carbonate sediment. Diagenetic alteration is not evident.