Claybanks State Preserve

Claybanks Forest is a 56-acre preserve featuring rock outcrops with remarkable, internationally known Devonian fossils. It is located 5 miles southwest of Nora Springs in eastern Cerro Gordo County. Consisting of three separate parcels of land, it was purchased by the Cerro Gordo Conservation Board in 1966. The site was dedicated as a geological state preserve in 1982. 

About the Land

This preserve is known historically as “Hackberry Grove.” The name “Claybanks” refers to the light-colored soft limey shales of the 375- million-year-old rocks bearing the fossils. These outcrops of soft shale and limestone found along the banks of the Winnebago River (originally called Lime Creek) are where the “Lime Creek Formation” was first described in the geologic literature and thus represent the “type-­locality” for this formation. 

The Lime Creek Formation is divided into three members: Juniper Hill, Cerro Gordo, and Owen. The relatively soft limestones and shales of the Cerro Gordo and overlying Owen Members are exposed here, and the abundant fossils within these layers comprise the famous Lime Creek Fauna, also known historically as the Hackberry Fauna. This unit is regarded as one of the premier fossil collecting beds in the United States.

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 Zinnia Avenue and go south 1.1 miles to Claybanks Drive. 
4. Turn west (right) and go 1.5 miles to Wren Avenue. Turn south (left), and drive 0.25 mile, crossing over the Winnebago River. 
5. The eastern unit of the preserve is on the west side of the road just south of the river (sign: Claybanks Forest—Cerro Gordo County Conservation). 
6. To get to the western unit, continue south on Wren Avenue to 225th Street. 
7. Turn west (right) and go 0.7 mile to the preserve entrance on the north side of the road (sign: Claybanks Forest—Cerro Gordo County Conservation). 
8. The middle unit is not accessible to the public. Do not cross private property.

Environment & Fossils

A variety of brachiopods (shellfish) and molds of gastropods (snails) and pelecypods (clams) can be found, 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”), chambered shells of cephalopods (squidlike), some trilobites (three-lobed), and numerous microscopic invertebrates. 

The lower part of the Owen Member can be seen at the top of the exposure and is between two and five feet thick. This more durable limestone also contains abundant fossils, with branching stromatoporoids, as well as worm tubes and burrows, crinoid debris, clams, gastropods, brachiopods, and other microfossils. 

This forested preserve contains large basswoods with some maple and hackberry on a flat upland. Steep slopes occur along the south bank of the Winnebago River. The high bluffs reveal natural exposures of the shallow, tropical sea deposits. 

Unlike other preserves, collecting of the exceptionally well-preserved and diverse marine fossils is permitted. The fossils weather free from the soft shales and limestones year after year. Claybanks Forest is fenced but available for hiking and hunting. Please do not trespass on private property. 

Other geological sites in the vicinity include Bird Hill State Preserve and Rock Fossil and Prairie Park.