Bird Hill State Preserve

Bird Hill is a 1-acre preserve featuring a fossil-rich outcrop of soft limestone and shale. It is located on the south side of 190th Street, eight miles southwest of Nora Springs in eastern Cerro Gordo County. It was once known as “Fossil Hill” because of the fossils found here. It was purchased by the Cerro Gordo County Conservation Board from the Bird family, after whom the area was named. This roadcut was dedicated as a geological state preserve in 1984 to recognize its diverse, abundant, well-preserved marine fauna and its historic significance as a collecting site. This site is one of three primary reference areas for the world-renowned Lime Creek fossil fauna of northern Iowa. 

About the Land

The Devonian (375-million-year-old) Lime Creek Formation consists of lime-rich deposits left in an ancient tropical sea. This geologic formation was named and described for natural exposures along the Winnebago River, which was originally called “Lime Creek.” The Lime Creek Formation is divided into three members (see figure): Juniper Hill, Cerro Gordo, and Owen. The soft limestones and shales of the Cerro Gordo and overlying Owen Members are exposed here, and the abundant fossils in these strata provide an important record of the marine life during Upper Devonian time. 

Directions

1. From the intersection of I-35 and Highway 18 southwest of Mason City, drive east on Highway 18 for 16 miles to County Road S70 (Zinnia Avenue). 
2. This local road is on the border between Cerro Gordo and Floyd Counties just southwest of the town of Nora Springs. 
3. Turn south (right) onto S70 (Zinnia Avenue) and go south 5 miles to 190th Street. 
4. Turn west (right) and go 0.25 mile to the parking area; the preserve is across the road- the rock outcrop on the north side.

Fossils & Geology

A variety of fossils, including brachiopods (shellfish) and molds of gastropods (snails) and pelecypods (clams) are found here, as well as solitary horn corals, colonial corals, and stromatoporoids (sponges). In addition, bryozoans (“moss animals”) and echinoderm (starfish family) debris can be seen, along with crinoids (“sea lilies”), the chambered shells of squidlike cephalopods, some trilobites, and numerous microscopic invertebrates. About 2.5 feet of the basal portion of the Owen Member, a more resistant limestone, is seen at the top of the exposure. The Owen also contains abundant fossils. 

The exceptional fossils at this site are continually exposed by natural weathering processes, and collecting is permitted. Other related geological areas in the vicinity include Claybanks Forest State Preserve and the Fossil & Prairie Park State Preserve.