Signal Butte EarthCache
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In order to count this Earthcache as a find, you must complete the following tasks and email the answers to me.
1. Estimate the height of the butte.
2. Describe the different layers of sediment that can be seen on the butte. Using the colors, try to match the sediments with their geological names listed in the description below.
3. Describe the effect erosion has had on the butte.
This Earthcache is located in the public right away just off of highway 212 in one of the most desolate areas of South Dakota. As you make your journey across this part of the state, you pass rather uneventful landscape; however, few gems, such as Signal Butte to the north, stand out among the monotony of the landscape and rocks. There is little civilization in this part of the state, so if you are traveling this direction, please make sure you have plenty of fuel and water. Enjoy!
The area in which Signal Butte sits is a bit of an anomaly. Spanning only a few miles in any direction, Signal Butte rests on an island of the White River formation, surrounded by a sea of Hell Creek formation. Signal Butte is an erosional remnant of the White River sediments that once covered the entire region. The rocks of the White River age have been known for many years to form numerous isolated buttes and mesas in the northwest region of South Dakota. Signal Butte is one example of such butte.
Signal Butte has been a victim of much erosion. The erosion of the Butte exposes different sediments that were deposited during various geological periods.
About 100 million years ago, North America was split into two haves by a large inland sea that extended over 600 miles wide and 2000 miles long. The entirety of South Dakota, at this time, lay at the bottom of this sea. While the inland sea was in existence, layers of sandstone and shale were deposited in the shallow sea water. These sediments make up the Ludlow formation, which forms the base of Signal Butte.
On top of the Ludlow formation lies the Tongue River formation. This formation consists of brown-to-tan clay. Much of the Tongue River formation, however, has disappeared due to erosion.
The next few layers comprise the majority of Signal Butte and are collectively called the White River group. About 40 million years ago, streams in northeastern South Dakota were responsible for depositing sediments on land rather than carrying them away to the sea. The reason for this was due to decreasing rainfall that deprived streams of much of their flow. These sediments are sands and clays with a large content of volcanic ash. The White River group comprises the majority of sediments you see exposed in Signal Butte.
The deepest layer of the White River group is Chadron Formation. In this layer, one can see, at the bottom, golden brown beds of iron-stained clays. This layer is anywhere from forty to seventy feet thick. The middle layer is comprised of white beds of coarse white sand and conglomerate between ten and forty feet thick. The uppermost layer is made of greenish clays. The thinnest layer, these clays are between seven and fifteen feet thick.
The Brule Formation, the uppermost White River group layer, lies on top of the Chadron and appears as tan-to-pink clay and silt. Intermixed with this sediment are small nodules of chalky rock and sandstones.
Capping Signal Butte is the Arikaree formation. This consists of fine white sandstone mixed with volcanic ash. Geologists conclude that this volcanic ash was wind-transported across the continent from an eruption occurring in the Western Cascades of Oregon and Washington.
Today, only small remnants of the White River Group survive on buttes in northwestern South Dakota. Signal Butte is one area where these sediments can be seen. To the north, the Slim Buttes also expose the White River Group sediments, as do some buttes in the Badlands further south and other buttes to the west.
NOT A LOGGING REQUIREMENT: Feel free to post pictures of your group at the area or the area itself - I love looking at the pictures.
Resources:
Gries, John Paul. Roadside Geology of South Dakota. Mountain Press: Missoula, Montana, 1996.
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