Niobrara Chalk Fort Hays Limestone Member.- The Fort Hays Limestone Member is 55 to 75 feet thick in the field-trip area, the latter figure being near the maximum for the west-central Kansas outcrop. The unit is characterized by thick to very thick beds of relatively resistant chalky limestone that is light gray to medium gray or light olive gray where fresh but more commonly weathered yellowish gray, grayish or pale grayish orange, or nearly white. The rock is mostly homogeneous micrograined carbonate rock composed largely of microspar, foram- inifers, and calcareous nannoplankton remains, but with locally abundant Inoceramus prisms and shell fragments of Inoceramus and Ostrea. Except for the basal beds, the rock is com- posed of 88 to 98 percent calcium carbonate (Runnels and Dubins, 1949, p. 17). The Fort Hays is conspicuously marked in many beds by irregular-shaped burrow structures that were completely backfilled with cuplike increments of chalky sediment. Such structures are most evident in little-weathered rock or on solution-etched surfaces. Slender cylindrical burrow structures, filled with pyrite, limonite, or visibly crystalline calcite, are common in the upper half of the member at Stop 9. Large burrows crisscrossed by grooves made by clawlike appendages of arthropods have been collected from the Fort Hays at several localities. One bed at localities 9 and 10 is in part thinly laminated to gently cross-laminated. Except for the several kinds of burrow structures, remains of benthonic macroinvertebrates in the Fort Hays are of drastically limited diversity, consisting primarily of Inoceramus deformis Meek and Ostrea congesta Conrad, the latter usually encrusting the former. Because isolated bowl-like valves of Inoceramus deformis are generally oriented concave upward, the attached specimens of Ostrea are preserved "facing" downward. Articulated valves of Inoceramus deformis are not commonly preserved in the articulated position. The only other common macroinvertebrate fossil in the Fort Hays is a species of Serpula. The Fort Hays Limestone Member lies upon the Codell Sandstone Member of the Carlile Shale with regional unconformity. In Colorado, several fossil zones lie between Carlile beds con- taining Collignoniceras hyatti (the youngest Carlile zone recognized in west-central Kansas) and Fort Hays beds containing Inoceramus deformis. Because of the gradational nature of Blue Hill and Codell strata and limited development of the Codell across much of central Kansas, the poorly fossiliferous Codell is not believed to represent all of the missing zones in west-central Kansas. The missing strata, if ever deposited, were removed before Niobrara deposition, but there is little relief and no unequivocal evidence for subaerial erosion at the unconformity. Apparently the sea floor lay above wave base for an extended interval of time following westward regression, which is reflected in sediments of the Carlile Shale. The writer (Hattin, 1962, p. 124) has postulated renewed transgression along the distant eastern margin of the Western Interior Sea while the west-central Kansas area remained above wave base. By the time deposition of the Niobrara began, the central Kansas region was far beyond the reach of terrigenous detritus being carried to the sea from the east or northeast. 20