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.
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).
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.
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.
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.
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