Chalky limestone beds of the Fort Hays, like those in the Greenhorn, are believed to reflect deposition in well-circulated water with concomitant agitation of fine carbonate mud at relatively shallow depths. Disarticulated, overturned, and fragmentary shells of Inoceramus deiformis, broken and disarticulated shells of Ostrea congesta, and local cross- laminations attest the mechanical disturbance of Fort Hays sediment. Ubiquitous and locally profuse burrow structures indicate a possible mechanism for the homogenization of Fort Hays sediment, and manifest an extensive infauna not represented in the bottom fauna during later deposition of the Smoky Hill Chalk Member. Smoky Hill Chalk Member.- The Smoky Hill Chalk Member ranges approximately from 560 to 620 feet in thickness along the Smoky Hill River in the field-trip area. The unit is domi- nated by light-olive-gray to olive-gray shaly chalk that is speckled by nearly white spheroidal calcareous pellets. The chalk is commonly weathered grayish orange to pale grayish orange, or dark yellowish orange to very pale yellowish orange, or yellowish gray. The rock apparently loses prominent bedding features and becomes harder and more massive during intense weathering. Such hard rock caps pinnacles and cliffs in many areas of badlands developed in the Smoky Hill Chalk Member. Bentonite seams are prominent features of the member and can be used, in groups, to effect regional correlation (V.B. Cole, oral communication). During Smoky Hill deposition the sea floor was apparently deeper, and the water quieter and less well circulated than during Fort Hays deposition. Burrow structures, so common in the Fort Hays, are virtually absent in the Smoky Hill, probably because the oxygen supply was insufficient for a highly mobile infauna. In addition, the immense size of Inoceramus in the Smoky Hill may reflect extraordinary development of respiratory tissue in response to poorly oxygenated water*. Similar huge specimens of Inoceramus are known in rock of the same lithology in the Greenhorn and Fairport. The lower part of the member is characterized by Inoceramus grandis (Conrad) and Inoceramus platinus Logan ranges through most of the member. The only other common benthonic macroinvertebrate in these rocks is Ostrea congesta, and in much of the section even these are rare. Specimens of Stramentum and Serpula are rare elements of the benthonic macroinvertebrate fauna. Ostrea is found most commonly en- crusting one or both valves of Inoceramus. The latter must have been recumbent during life because (1) none occur erect in the sediments, (2) there was no suitable substratum to which the large shells could have been attached, and (3) growth form of attached oysters suggests a recumbent position for Inoceramus. Occurrence of Ostrea on both valves of many Inoceramus specimens proves that some of the inoceramids were overturned, apparently during life, possibly by fish or by occasional storm activity, or maybe even by the clam itself. Despite seemingly low oxygen content of the water, the probability of overturning by fish cannot be discounted, especially in light of the abundance of shell-crushing shark teeth that have been collected in the Niobrara. Fragments of Inoceramus and Ostrea and disarticulated valves of Inoceramus are common in these rocks and may be evidence of predation by such fish. *However, large species of Inoceramus are characteristic in rocks of this age elsewhere in the Northern Hemisphere, and some large specimens are known in rocks representing a closer- to-shore environment than that in which the Smoky Hill beds were deposited (W.A. Cobban, written communication, 1965). 21