Figure 2.- Major structural features reflected in Cretaceous rocks in the field-trip area. Named minor structures are: (1) Fairport-Natoma Anticline, (2) Pfeifer Anti- cline, (3) Alanthus Dome, (4) Hell Creek Structure, (5) Elkader Dome, (6) Twin Buttes Anticline, (7) Ellsworth Anticline. Adapted from Merriam (1963, p. 178). Named minor structures expressed in Cretaceous rocks in the field-trip area include the asymmetrical Fairport-Natoma Anticline in northeastern Ellis County; the Pfeifer Anticline of southeastern Ellis County, aligned with the Fairport-Natoma structure; Alanthus Dome in southeastern Gove County; the domelike Hell Creek structure in southern Gove, southeastern Logan and northeastern Scott counties; Elkader Dome in southeastern Logan County and Twin Buttes Anticline in southwestern Logan County (Jewett, 1951). Merriam (1963) also included the Ellsworth Anticline of northwestern Ellsworth County among minor structures reflected in Cretaceous rocks. The regional dip of the Cretaceous is further modified by numerous unnamed minor anticlines, synclines, and other structures which have been described by Lupton and others (1922), Rubey and Bass (1925), Bass (1926), Elias (1931), and Johnson (1958). On a smaller scale, hundreds of normal faults that are best expressed in the Niobrara Chalk occur also in the Carlile and Pierre Shales. In a small area of northwestern Ellis County, Bass (1926, p. 44) mapped 76 faults, nearly all of which are in the Smoky Hill Chalk Member. Maximum displacement observed by Bass was 80 feet and the greatest length of trace among these faults was less than half a mile. Johnson (1958, p. 30) believed that displacement along some such faults in Logan County might exceed 200 feet and mapped the trace of one fault, or closely spaced set of faults, that is nearly 3½ miles in length. The Pierre Shale lies in fault contact with the Niobrara Chalk in a number of areas in Logan and Gove counties. Most of the faults in the field-trip area have dips of 45° ± 10°, although some are nearly vertical, and are marked by masses of slickensided calcite that may be very thin or several inches thick; but brecciation has occurred along some fault 8