Effigy Mounds National Monument: Paleontological resource inventory (public version)

Tweet, Justin; Santucci, Vincent

doi:10.36967/2307451

Effigy Mounds National Monument (EFMO) in northeastern Iowa was proclaimed on October 25, 1949, to protect a significant concentration of Native American burial mounds, many of which were built in the shapes of animals such as bears. Many of these effigy mounds are found atop commanding bluffs flanking the Yellow River at its junction with the Mississippi River. The geology of the monument has not been a central interest in its history, but the rocks contain evidence of millions of years of shallow marine life from the early Paleozoic. Paleozoic fossils were first documented at what is now EFMO in the mid-19th century, with few other reports. This inventory is the first report to focus on the paleontological resources of EFMO, documenting the scope, significance, distribution, and management issues associated with these non-renewable resources. It is based on two field visits that took place in July and November 2023. The bedrock geology exposed within EFMO includes several rock formations dating from between roughly 490 and 450 million years ago, from the end of the Cambrian into the Late Ordovician. These formations capture parts of two major cycles of marine transgression and regression, with a depositional hiatus between them. These rocks were deposited in and near shallow continental seas at a time when the Upper Midwest was at tropical latitudes south of the equator. A variety of fossils have been observed in the Jordan Formation (late Cambrian age), Prairie du Chien Group (Early Ordovician), and Platteville Formation (Late Ordovician). Most of the fossils in the two older units are burrows and other trace fossils. The Platteville Formation includes fossils of a much broader array of life. More than two dozen distinct fossil forms, most identifiable to a genus, have been recognized from the Platteville Formation at EFMO. The fossils belong to a typical Platteville assemblage of horn corals, bryozoans, brachiopods, bivalves, nautiloids, gastropods, trilobites, ostracodes, crinoids, and others. EFMO also has a small Holocene record from Founders Pond that is useful for reconstructing the monument’s ecology during the time when the mounds were built. In addition to the above, this report documents cultural associations with paleontological resources and the monument’s museum collections (including the extensive fossil collections of foundational figure Ellison Orr). Discussion and recommendations for research, interpretation, and paleontological resource management are included in this report.

Effigy Mounds National Monument: Paleontological resource inventory (public version)

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