. Bulletins of American paleontology. Text-figure 8.—Exposures of the entire Mason City Member at Cooper's Bend (Locality 19. Appendix). The massive dolomites forming the shelf at the base of the sequence are underlying Cedar Valley beds, with m (4 ft) of Mason City divided into lower shaley. rubbly coral- bearing limestones and upper more massive unit, in turn overlain by slope-forming shaley beds of the Rock Grove Member under trees. Photograph taken in June. I Exposure - Shell Rock Fm. Aureola 7 1/2 Min. Quad. 1 Mile Text-figure 9.—Exposures of the Shell Rock Formation along the
. Bulletins of American paleontology. Text-figure 8.—Exposures of the entire Mason City Member at Cooper's Bend (Locality 19. Appendix). The massive dolomites forming the shelf at the base of the sequence are underlying Cedar Valley beds, with m (4 ft) of Mason City divided into lower shaley. rubbly coral- bearing limestones and upper more massive unit, in turn overlain by slope-forming shaley beds of the Rock Grove Member under trees. Photograph taken in June. I Exposure - Shell Rock Fm. Aureola 7 1/2 Min. Quad. 1 Mile Text-figure 9.—Exposures of the Shell Rock Formation along the Shell Rock River northwest of Greene. Iowa. Further details of stratigraphic localities are noted in the Appendix. tostrophia "faunule", cm thick, which is one thin bed characterized by the small, concavo-convex bra- chiopod Nervostrophia). Such units are only locally recognizable, and are not of use as zones. Text-figure 23 shows lithologic correlations of Cerro Gordo units within sections described between Owen Grove and Rockford. Several marker beds aid in the correlation and rec- ognition of portions of the member, often allowing pre- cise placement of a very limited outcrop sequence within the stratigraphic framework of the entire unit. Remarkable uniformity exists within this member in its area of outcrop, facilitating correlation. One such bed is the "rusty" bed, shown at the base of the upper unit ("Spirifer" zone of Fenton, 1919), at the type section in the bluffs along the Winnebago River (Lo- cality 28, Text-figs. 16,17). This bed is hematitic, due to the oxidation of pyrite on what apparently was a hard ground. It is characterized at the type section by a great abundance of stromatoporoids and stromato- poroid-coated colonies of Pachyphyllum. Hexagonar- ia, lowaphyllum, and the tabulate genus Alveolites. The bed can be recognized as far west as roadcuts along County Highway D, south of Portland, Iowa (Locality 27, Text-fig. 16),
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