planes. Dip on beds directly adjacent to such faults was found to be as high as 15° by Johnson (1958, p. 30) and 19° by Bass (1926, p. 45). These faults are not believed to be related to regional structure. In contrast to Cretaceous rocks, the Ogallala Formation is deformed only locally (Elias, 1931). Structure at the top of the "Algal limestone" marking the top of the formation is characterized by "monotonously uniform east dip, which reflects none of the major structural features in western Kansas" and averages approximately 15 feet per mile between the Colorado line and Mitchell and Lincoln counties (Merriam, 1963, p. 197). GENERAL STRATIGRAPHY Classification and characteristics of all stratigraphic units that crop out in the field-trip area are summarized in Table 1. Along the valley of the Smoky Hill River and its western tributaries exposed Upper Cretaceous marine rocks reach an aggregate thickness of approximately 1,670 feet. Of this thickness, about 600 feet are dark-gray noncalcareous shale belonging to the incompletely exposed Pierre Shale. The Colorado Group is approxi- mately 1,060 feet thick, including 850 feet of limestone and chalk, 200 feet of dark-gray noncalcareous shale, 6 feet of quartzose sandstone and siltstone, and approximately 8 feet of bentonite. Sandstone locally comprising the uppermost 5 feet of the Dakota Formation is probably also of marine origin. Along the Saline River valley, the Colorado Group is approximately 995 feet thick, with 740 feet of limestone and chalk, 215 feet of noncal- careous shale, a maximum of 31 feet of sandstone and siltstone, and approximately 8 feet of bentonite. In addition, at least 22 feet of sandstone in the upper part of the Dakota Formation locally are of marine origin. Cretaceous strata were truncated regionally by pre-Pliocene erosion, then buried beneath Ogallala deposits which, at one place or another in the State, rest on every one of the Cretaceous formations. Pleistocene and Recent deposits rest on Cretaceous rocks in many places, especially along the valleys of larger streams where post-Pliocene erosion removed the widespread mantle of Ogallala sediments. Common macroinvertebrate fossils in Upper Cretaceous marine units of western Kansas are illustrated in Figures 3-5. Photomicrographs of representative thin sections of Cretaceous rocks are illustrated in Figure 6. DESCRIPTIONS OF CRETACEOUS UNITS IN FIELD-TRIP AREA Dakota Formation Only the upper or Janssen Clay Member of the Dakota Formation is exposed along the route of the field trip. This unit is 52 feet thick in the type area in Ellsworth County. Lithology changes abruptly and complexly both vertically and laterally in this unit. Five general rock varieties include more or less evenly bedded and generally highly ferruginous 9