The Walnut Formation, also called the Walnut Clay, is Lower Cretaceous (Albian Stage). It is a marine formation in which bivalves, gastropods, ammonites, and echinoids are common. A good nontechnical reference is C. Finsley, 1996, "A Field Guide to Fossils of Texas," 2nd Edition, Gulf Publ. Co., Houston, TX. For the ammonites, my primary reference is B. L. Emerson et al., 1994, "Texas Cretaceous Ammonites and Nautiloids," Texas Paleontology Ser., Publ. No. 5, Paleontology Section, Houston Gem and Mineral Society. I have also used K. Young, 1966, "Texas Mojsisovicziinae (Ammonoidea) and the Zonation of the Federicksburg," Geol. Soc. America Memoir 100.
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