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On the bluffs above the Mississippi in northeastern Iowa, people of the Late Woodland period shaped the earth into bears and birds. Effigy Mounds National Monument protects one of the largest concentrations of ancient burial mounds in the United States — and the rock beneath them is older still, laid down in Cambrian and Ordovician seas.
The Marching Bear Mounds.
| Where | on the Mississippi about 45 miles (72 km) northwest of Dubuque, Allamakee and Clayton Counties, Iowa |
| Proclaimed | October 25, 1949 |
| Size | about 2,526 acres (1,022 hectares) |
| Mounds | 206 mound sites, 31 of them effigies shaped like birds or bears |
| Built | Late Woodland period, between about 450 BCE and 1300 CE |
The mounds
The mounds offer a view into the social, spiritual and ceremonial life of the people now called the Effigy Moundbuilders; they are thought to honour the dead and to have been sacred places. The monument has four units:
- North Unit — the headquarters and visitor centre, Little Bear Mound, Great Bear Mound (the monument's largest effigy) and many mound groups.
- South Unit — the Marching Bear Group, ten bears and three birds in a line.
- Heritage Addition (added in 2000) — five known mounds, five ancient stone quarries and four historic sites.
- Sny Magill Unit — about 10 miles (16 km) south of headquarters, with more than 100 mounds, the densest group known in the region.
The rock underneath
The bedrock is sedimentary: Cambrian and Ordovician layers, cut through by the Yellow River and its tributaries, which shape the North and South Units and the Heritage Addition.
| Age | Formations | Where |
|---|---|---|
| Cambrian (oldest) | Jordan Sandstone, St. Lawrence Formation, Lone Rock Formation | exposed along the Yellow River |
| Ordovician | Oneota and Shakopee formations, St. Peter Sandstone, Glenwood, Platteville and Decorah formations | the valley sides of the Yellow River; the Sny Magill Unit sits entirely on St. Peter Sandstone |
| Ordovician (youngest here) | Dunleith Formation | the western Heritage Addition and southern South Unit |
The monument lies in the Central Lowland province and shares its geologic story with a much wider region.
Across Blue Gill Pond to Fire Point.
Maps and reports
The Park Service's Geologic Resources Inventory has produced a digital geologic map, a report and a poster for the monument, and its soil survey is complete; all are free on the NPS DataStore.
Sources
Written from "NPS Geodiversity Atlas — Effigy Mounds National Monument, Iowa," National Park Service Geologic Resources Division, a work of the United States government in the public domain, which cites the National Park Service (2014, 2016) and O'Bright (1990).
I disse samlingerEffigy Mounds National Monument
Licens: CC0 1.0 (offentligt eje) · Bearbejdet efter www.nps.gov
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