The Mountains of Southern Illinois EarthCache
The Mountains of Southern Illinois
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I grew up in Southern Illinois and still think it’s a beautiful place to see. There are a lot of scenic areas especially in and around the Shawnee National Forrest. Garden of the Gods is probably one of my favorite places to visit. I have driven by this area many times in years past, looked at the “mountains” in the distance and wondered how and why they are here.
Let me first say that there is no need for you to leave the roadway to do this earthcache. This isn’t a heavily traveled road but please pull safely to the side. The given coordinates take you to an informational sign which will provide you with some of the answers to the questions you need to e-mail me. The remaining information you will need can be found on cache page. Please do not put the answers in your log. I think this cache would be handicap accessible.
I’m going to list the questions early in this description as some of the newer GPS’s don’t support the entire cache description. However, please read on and get the geological explanation for these “mountains” so that you will have successfully experienced the earth cache and learned something.
These are the questions you will need to answer:
1. From left to right, what are the names of the summits you see?
2. Which summit is the highest and what is it’s height?
3. What three Illinois counties are part of the Shawneetown Fault?
4. What phenomenon is responsible for creating nearby Hick’s Dome?
From the coordinates, you will be able to look in the distance and view the “mountains of southern Illinois”. The largest summit is the steepest rise above common plain in Illinois. The informational sign notes this summit is the 2nd highest point in the state of Illinois. The summits you see were a result of seismic activity, which raised the area 700 feet somewhere between 500 and 130 million years ago. Seismic activity of an area refers to the frequency, type and size of earthquakes experience over a period of time. Earthquakes are the result of a sudden release of energy in the earth’s crust. They can be caused by rupture of faults, which is the most common cause, but also by other events such as landslides, mine blasts, and volcanic activity.
The area that you see sits aside the Shawneetown Fault. By definition, a fault is a crack in the earth’s crust resulting from displacement of one side with respect to the other. This particular fault occurred between Early Permian and Late Cretaceous period. The main crustal blocks moved vertically but returned to roughly their original position and show little offset across the fault zone. There are, however, individual faults with displacements up to 3500 feet. The Shawneetown Fault stretches east-west in southern Gallatin and eastern most Saline counties to south-southwest in southern Saline and northeastern Pope Counties.
Southern Illinois sits on the border of two major fault systems: The New Madrid Seismic Zone and the Wabash Valley Seismic Zone. There is some speculation by geologists that the Shawneetown Fault may be part of the New Madrid System. The New Madrid Fault was responsible for some of the most powerful earthquakes to hit the contiguous United States in 1812. Following a 1968 Illinois Earthquake, scientists discovered an unknown fault under Saline County called the Cottage Grove Fault. This fault is the result of a small tear in the Earth’s rock.
To the east of the ridge of mountains you see is Eagle Valley which was formed by a syncline, which is a downward folding of the earth’s crust. Synclines are recognized by a series of rock layers that grown progressively younger and involve the youngest layer at the fold’s center and by a reverse sequence on the other side of the center (or hinge). These folds occur during crustal deformation resulting from compression that accompanies mountain building.
Earlier it was mentioned that earthquakes can be caused by volcanic activity. Southern Illinois is hardly the place you would expect to find a volcano. North east of nearby Herod, IL you will find Hick’s Dome, an area of sedimentary rocks and unusual surface formations. Hick’s Dome was caused by a volcano 150 – 250 million years ago. The volcano never broke the surface but created a crypto-volcanic implosion (a violent collapse inward). This phenomenon was responsible for the fluorspar, zinc, lead, and silver that is found in the area. The volcanic shape of Hick’s Dome can be observed from the sky, per pilots flying over the area.
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