New information concerning Cambrian and Lower Ordovician stratigraphy in Wyoming and Montana is derived from 26 measured sections in uplifts along a 430-mi trend from the Rawlins uplift on the south, through the Ferris Mountains, Rattlesnake Hills, Bighorn Mountains, Prior Mountains, Big Snowy Mountains, and Little Rocky Mountains on the north.

Seas transgressed east-southeastward up a paleoslope of low regional relief but with several hundred feet of local relief. The sedimentary record consists of a series of lithotope belts each extending generally parallel with the shoreline. From the shore seaward the order is as follows: (1) sandstone, generally clean, medium- to coarse-grained, crossbedded and burrowed, deposited in beach, intertidal, and shallow subtidal zones; (2) sandstone, generally clean, fine-grained crossbedded, laminated and burrowed, deposited in the shallow subtidal zone; (3) sandstone, the same as (2) except a glauconitic greensand and may be interbedded with siltstone and shale; (4) siltstone, may be interbedded with fine-grained sandstone and shale; (5a) shale, usually burrowed, with micrite beds and nodu es, seaward from (4) in deeper water; (5b) shale and intraclast limestone conglomerates indicate shallower water and offshore intertidal zone environments; (6) laminated limestones, intraclast limestone conglomerates, and crinozoan calcarenites suggest an offshore shallow subtidal and intertidal environment.

The faunal record begins with the Bathyuriscus-Elrathina Zone of medial Cambrian age and ends with the Bellefontia Zone of Early Ordovician age. Trilobites and brachiopods dominate the faunas but conodonts assume major importance in latest Cambrian and Early Ordovician time. Both faunal-zone boundaries and key beds are isochronous surfaces that form planes of correlation within lithotopes and across facies boundaries between lithotopes.

The Bathyuriscus-Elrathina Zone and "lower" Bolaspidella Zone are of lithotopes (1) through (5). The "upper" Bolaspidella Zone reflects an abrupt east-southeastward shift in depositional environments and only lithotopes (3) through (5) are present. The Cedaria Zone is largely (4) and the Crepicephalus and Aphelaspis Zone are (5b). Seas withdrew after initial Aphelaspis Zone deposition and returned at about the beginning of the Elvinia Zone. Elvinia, Taenicephalus, Idahoia, and early Saukiella Zones are (5b) with the shale dominant at the base and limestone increasing upward. Late Saukiella, Mississquoia, and Bellefontia Zones are of lithotope (6).

An unconformity variably truncates the section and younger strata lie on rocks ranging in age from Bellefontia Zone limestones in the Big Snowy Mountains to Bolaspidella Zone greensands in the Rawlins uplift.

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