The Big Snowy group, as defined by Scott, consists of an upper and middle Mississippian series of shales, limestones, and sandstones with some evaporites. Recently drilled deep wells and new interpretations of older wells in eastern Montana and the Dakotas yield information which makes possible further considerations on the eastward extension of Big Snowy sediments.

An isopach map of the group indicates that an east-west depositional basin in Montana fingered over the northern portion of a widespread area of lower Mississippian (Madison) deposition. South of a narrow peninsula which occupied the approximate

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position of the Montana-Wyoming state line, a more restricted basin was the site of nearly simultaneous deposition of the Sacajawea formation of Wyoming.

Sediments of the Big Snowy group in eastern Montana differ from those of the type (surface) section in the diminution of coarse sandstone and green shale and the notable increase in red shale and anhydrite. The ease of divisibility of the Big Snowy group into its component formations rapidly decreases eastward, and, moreover, there appears to be a gradational transition from Madison into Big Snowy deposition in contrast to the presence of a hiatus between the two groups in central Montana.

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