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Western Interior Paleontological Society - Toadstool Park

Footprints in Stone: Trackways at Toadstool Park

By Steven Wade Veatch


Introduction
Stratigraphy of Toadstool Park
Trackways
Summary

Introduction

Toadstool Park is an interesting geological site in the Oglala National Grasslands, near the town of Crawford. In this quiet corner of northwestern Nebraska, rocks of Eocene, Oligocene, and Miocene age are magnificently exposed. The park features sandstone channel deposits that include the fossilized footprints of rhinos, giant pigs, camels, birds, and other animals. One of the longest known mammal trackways of the Oligocene Epoch are preserved in the park (Nixon and Lagarry 1993).

Varying hardness of the local rocks resulted in unequal erosion, leaving the more resistant channel sandstone precariously balanced on pedestals of softer rock. Early visitors, who thought the area's features resembled toadstools, named the park.

Around 30 million years ago, a broad, shallow river flowed through this hot and humid area, attracting prehistoric mammals that came to drink. The currents carried and deposited layer upon layer of volcanic material, sand, and silt. Over time, these deposits were cemented together to form rocks containing the fossil record of early Great Plains animals that roamed the area 30 million years ago. As the Cretaceous sea receded, the High Plains emerged, gradually becoming gently-rolling grasslands.

The "High Plains" orogeny, a period of plateau uplift culminating about 10 mya, brought about the Toadstool Park Fault. This normal fault, running approximately southwest to northeast with an offset of about 25 to 30 meters (600 to 800 feet), goes through the park (Leite, pers. comm.). This important uplift is what brought these rocks up 1,220 to 1,520 meters (4,000-5,000 feet) above sea level and is responsible for their rapid erosion today.

The relentless action of wind and water eroded the rocks into badlands, exposing this remarkable record of small horses, camels, huge land tortoises, gigantic pigs, rhinos, and other animals.

Stratigraphy of Toadstool Park

The stratigraphy (sequence of rocks) starts with the oldest Eocene age Chadron Formation, followed by the Oligocene Brule Formation, and then the Miocene age rocks of the Arikaree Group.

The Chadron Formation consists primarily of fine-grained, light-brown to pink claystones. The Chadron ranges up to 90 meters (300 feet) in thickness. These rocks were formed largely from widespread deposits of volcanic ash, now mostly weathered into clays. Channel sandstones can be found, along with occasional veins of opal and chalcedony (Leite, 1999). The Chadron rests on the Cretaceous Pierre Shale.

The Brule Formation, which overlies the Chadron, was named for the Brule Indians of western Nebraska and South Dakota. The Brule is the uppermost formation in the White River Group and is exposed over much of the West, including Nebraska, North and South Dakota, Colorado, and Wyoming (Leite, 1999).

The Brule varies in lithology, becoming finer grained toward its top and includes the Orella (lower) and Whitney (upper) Members.

The Whitney Member, up to 85 meters (270 feet) thick, is composed of massive siltstones interrupted only by a few ash beds (Schultz and Stout 1955). The Orella member, 60 meters (200 feet) thick, contains numerous beds of poorly sorted arkosic sands, interbedded with siltstones and claystones (Leite,1999). The Orella Member frequently weathers into a "knife-edge" form.

The southern part of the park contains the Arikaree Group, consisting of these three formations from the oldest to the youngest: the Gering, Monroe Creek, and Harrison Formations. Only the Gering, appearing as a white siltstone composed of volcanic ash, is exposed in the Toadstool Park area (Leite, 1999).

Formation Age
Chadron Formation Eocene
Brule Formation Oligocene
Arikaree Group Miocene
Table 1. Toadstool Stratigraphy

Trackways

Geologists refer to fossil tracks and trails as "ichnofossils." Largely ignored until about 20 years ago, these important fossils offer paleontologists a glimpse of animal behavior (Orr and Orr, 1998).

Many different kinds of animal tracks are preserved in the bedding planes of the Orella Member of the Brule Formation. The tracks are distinctive from the other depressions in the rock--they do not occur randomly, but have a degree of direction and orientation.


Figure 1. Trackway at Toadstool Park. Image by Michele Veatch

Much of the mammal community is possibly represented, including three-toed and four-toed animals, both large and small. Some of the tracks have been interpreted as being made by the large rhino, Subhyracodon. The tracks show that the rhino was walking along the river, probably browsing on plants. Something startled this animal, causing it to abruptly run downstream through the mud, using the channel as a path.

Even-toed tracks show the presence of enteledonts--giant wild pigs that were scavengers, following the herds of rhinos. The smaller rhino, Hyracodon, horses, and oreodonts (hoofed mammals distantly related to modern camels and swine) also walked along the river, leaving their tracks. The entire trackway, extending nearly 1.2 kilometers (three fourths of a mile), documents one of the longest migrations preserved in stone for this period of geologic time.

Summary

The sandstone channel deposits of Toadstool Park feature fossilized footprints of rhinos, giant pigs, birds, and other animals. Preserved in slabs of sandstones are tracks that show aspects of these ancient animal's behavior such as walking, running, and stopping to drink. This remarkable area gives visitors a glimpse back to the ancient beginnings of the Great Plains.

Figure 2. A train passes by at sunset near Toadstool Park.
Image by Gerald Sharp, used with permission.


Acknowledgments

I thank Beth Simmons, Metropolitan State College, who provided valuable comments on this paper.


References:

Leite, M. 1999. Field Trip to Toadstool Park, http://cscms.csc.edu/NaturalSci/TOADSTOO.HTM

Nixon, D. and Lagarry, H. 1993. New trackway site in the White River Group type section at Toadstool Park, Nebraska: paleoecology of an Oligocene braided stream, riparian woodland, and adjacent grassland. Journal of Vertebrate Paleontology 13 (supplement to number 3): 50 p.
Orr, W. and Orr, E. 1998. Oregon Fossils. Kendall-Hunt, Dubuque, 390 p.

Schultz, C. B., and Stout, T. M. 1955. Classification of Oligocene sediments in Nebraska. Bulletin of University of Nebraska State Museum 4: 17-52.

© Steven Veatch 2000

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