. Bulletin. Natural history; Natuurlijke historie. 16 STRATIGRAPHY OF THE THERMOPOLIS SHALE designated accurately, it is not now possible to ascertain whether the term re- ferred only to channel sandstones in the upper part of the Cloverly, or to sand- stone beds at the base of the rusty beds, or to some of each. Neither the Cloverly nor the rusty beds' sandstones, however, would constitute a unit that would be practical to map, and the inclusion of all the sandstones in this general interval in one unit would involve ignoring the significance of the regional boundary between nonmarine sedimen
. Bulletin. Natural history; Natuurlijke historie. 16 STRATIGRAPHY OF THE THERMOPOLIS SHALE designated accurately, it is not now possible to ascertain whether the term re- ferred only to channel sandstones in the upper part of the Cloverly, or to sand- stone beds at the base of the rusty beds, or to some of each. Neither the Cloverly nor the rusty beds' sandstones, however, would constitute a unit that would be practical to map, and the inclusion of all the sandstones in this general interval in one unit would involve ignoring the significance of the regional boundary between nonmarine sediments and marine and marginal marine sediments which is marked by a zone of spherulitic siderite. Hence, the term Greybull should not be used beyond the confines of the Greybull oil field where originally it was in- formally applied to an oil sand. Recognition of the Muddy sandstone as a formation and concomitant re- definition of the term Thermopolis to apply only to the shale unit below the Muddy leaves the soft black shale unit, which was originally included by Lupton (1916, p. 168) in the upper part of his Thermopolis shale, without a name. What should it now be called? Two courses are open: one would give the unit a new name; the other would include it in the Mowry shale, thus extending the term Mowry down to the top of the Muddy sandstone. As a guide to the most desirable course, we might examine what has been done in the past with the soft black shale unit whenever the Muddy was considered to be of formation rank. Most workers who have considered the Muddy a formation have followed Love's (1948, p. 113) suggestion that the term Mowry be extended downward to include the soft black shale sequence. Love first applied this classification in the. WIND RIVER y' BASIN ^ / 50' 0' I .*_. M N I Figure 3. Isopach map of Shell Creek shale in eastern Wyoming, showing decrease in thickness over the mountains which border the Big Horn Basin on the east and Please note that these i
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