WARNING: DO NOT ATTEMPT TO CLIMB ON THIS OUTCROP
The Rock Wall is Unstable
This EarthCache is located south of Bloomington NE on E Rd., and east of 27 1/2 Rd. It is located on a public gravel road. No permission is required to visit this location. Parking is along the shoulder. Use flashers while you are parked.
REQUIREMENT FOR LOGGING THIS EARTHCACHE
Email the answers to lyric.soprano and do not post them with your online FIND LOG. You will have to visit the site to qualify for this EarthCache as online research will not provide you with all the answers. Please post a picture of your visit to this site (optional). The difficulty rating is based on the requirement to log this cache and the number of answers required. The terrain rating is based in part for the distance required to get to this location.
1) Measure the length of this outcrop from POINT A to POINT B.
2) What colors of Rock are visible at this outcrop.?
3) What is the reason for the different colors?
4) Look for fossils. If you find any describe your find.
5) Estimate the highest point of this outcrop.
6) Why is NIOBRARA CHALK A sedimentary rock?
7) How long ago was the CRETACEOUS AGE?
GEOLOGY OF THE NIOBRARA FORMATION
“This is a sedimentary rock unit known as the Niobrara Chalk. A chalk is a sedimentary rock made out of primarily the shells of small one-celled organisms. The shells are made out of calcium carbonate. All these pieces are only loosely held together and so when you rub it on rock or a chalkboard the small white particles come off and make a streak. The Niobrara Chalk is technically more of a marl than a chalk, which simply means there is also some clay mixed in with the calcium carbonate. These marls were deposited in an arm of a sea that used to extend into the continent from the Gulf Coast, and this sea is known as the Western Interior Seaway. They are of Cretaceous age, the same time as the large dinosaurs were roaming about.
The rock specimens you will find at this location are primarily the marl/chalks, but there very different colors and appearance is due to surface weathering transforming the original rock. When unweathered the Niobrara Chalk is full of organic material, which gives it a dark coloration. The organics are the first to go when the rock is weathered by exposure to water and atmospheric gases (the organic material basically is either eaten by microbes or is oxidized). This turns the rock white (the color of calcium carbonate) when all of the organic material has been removed, and partial removal results in all these different shades of grey. Some of the chalk has a small percentage of iron pyrite in it, which when it weathers oxidizes basically to natural rust, which can give the orange and yellow tints to the chalk. There is one different rock type in the cliff. Some thin white layers are volcanic ash beds which through alteration and burial have been changed primarily into bentonitic clays. The rest is all the Niobrara chalk.” [from email corrispondence with Harmon D. Maher Jr., PhD., Dept. Geography and Geology. University of Nebraska- Omaha, Nebraska. November 4, 2014].
THE CRETACEOUS AGE AND SEDIMENTATION
The Cretaceous Age is part of the Mesozoic Era occurring 65 to 145 million years ago. “The Niobrara formation exposed in this cliff is quite rich in organics. It served as a carbon sink during the Cretaceous time as billions of tiny organisms extracted carbon dioxide from the sea and the atmosphere to build calcium carbonate skeletons (the chalk) and their bodies (the organic material). Upon dying, they sand to the seafloor and the accumulating sediment locked the carbon in place. In the middle of the Western Interior Seaway, where clays, silts, and sands rarely reached, the majority of the sediments were the products of biological activity. This boom in the accumulation of carbonate and organic-rich sediments was part of a global phenomenon.” (Maher et. al., p. 165).
TOPOGRAPHY
This outcropping is a slice of the exposed hillside. If you look at the very top of the outcrop where these is plant growth you will notice the brown sandy soil that covers the Niobrara Chalk. This top layer is called loess. The loess may vary in depth due to erosion and wind distribution of the soft material. At the lower parts of the outcrop you may see evidence of a thin white layer of volcanic ash that were deposited from volcanic eruptions far to the west before the Niobrara sediment was deposited. You may find fish fossils and other types of fossils. However, many of the organisms here are a collection of microscopic organics that created the layers of rock seen here.
REFERENCES
Harmon D. Maher, Jr., G.F. Englemann, and R.D. Shuster. Roadside Geology of Nebraska. Mountain Press Publishing Co., 2003. pp. 164-169.
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