Nebraska's geological past covers a wide span of time dating
from roughly 300 million years ago. Layed down layer after layer,
our state's geology looks more like a cake, than anything else. The
oldest rocks in the layer cake are near or exposed at the surface
in the southeastern part of the state and date from the Late
Pennsylvanian epoch. Next comes a thin slice of rock from the
Permian Period (also in SE Nebraska), followed by rock from the
Early and Late Cretaceous Epochs that are found along the rivers in
Nebraska's northern and southern tiers and ranging from NE to south
central Nebraska. The most recent bedrock comes from the Eocene,
Oligocene, and Miocene epochs and covers Nebraska from the Sand
Hills on west.
Fossil evidence within these rocks indicate that Nebraska
experienced a variety of conditions ranging from shallow seas
(Pennsylvanian, Permian, and Cretaceous) to warm tropical
grasslands (Eocene, Oligocene, and Miocene). Some notable Nebraska
Earthcaches illustrate the ancient environment and provide examples
of the abundant fossils that can be found in Nebraska.
Toadstool Park is an interesting geological site in the Oglala
National Grasslands, northwest of the town of Crawford. At this
location, rocks of Eocene, Oligocene, and Miocene age are exposed.
The park features sandstone channel deposits that include the
longest fossilized trackway of animals of the Oligocene Epoch.
Unusual geologic features give the park its name--early visitors
thought they resembled toadstools, and the name stuck.
Panorama of Toadstool Geologic Park
These toadstools are actually evidence of paleorivers (i.e.,
ancient rivers). Wherever you see sandstone in Toadstool Park you
can know that an ancient river once flowed there. Around 30 million
years ago, fast flowing rivers from the Rockies to the west and the
Black Hills to north flowed through this area and deposited
coarse-grained sand which was cemented into sandstone over time.
While sand was accumulating in the river bed, silt was slowly being
deposited in the river's flood-plain. This silt, over time, became
siltstone (claystone in the field guide). The river, over hundreds
of thousands of years, changed course many times and left
alternating layers of sandstone and siltstone. 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.
A period of plateau uplift culminating about 10 million years
ago raised these rocks up 1,220 to 1,520 meters (4,000-5,000 feet)
above sea level resulting in their exposure and rapid erosion. All
the layers that had been built up over millions of years, began to
wear away. This process continues today. The hard sandstone, which
is more resistant to the forces of erosion than the softer
siltstone, often forms a protective cap over a stem of siltstone
below, forming a toadstool. These toadstools are just short-lifed
formations. The park's most famous toadstools collapsed over the
last several years, but other new toadstools continue to be formed
as the sandstone and siltstone erodes from the hillsides.
Image of a thick layer of sandstone atop siltstone
In order to log this Earthcache, you need to go to the following
locations, discover the information, and then provide the answers
in an email to me. NOTE: The trail guide available at the trail
head on the west side of the parking lot (N 42 51.461 W 103 35.071)
will provide MANY of the answers!
1. At N 42 51.547 W 103 35.316, you will notice several of the
fallen toadstools (knocked down during a severe storm several years
ago). What is the estimated thickness of the sandstone block
nearest the bench?
2. At N 42 51.528 W 103 35.334, what is the chemical name for
the dark, jagged gravel scattered about at your feet?
3. At N 42 51.377 W 103 35.347 you will be climbing up a small
hill. Before you reach the top, stop at these coordinates (roughly
stop #5 on the trail map) and examine the rock face directly east.
How many times did a river channel run through this rock face?
4. At N 42 51.383 W 103 35.196 there are unusual trackways in
the rock exposed at your feet. What two species of animals are
thought to have left prints in the mud 30 million years ago?
If you like, take a photo of one of the tracks and post it with
your log.
Permission was given by the Forest Service Manager to place this
Earthcache.