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A LONG tape measure is needed for this earthcache!!!
This is an earthcache which will hopefully provide you with an educational experience about a feature on our planet in AR.
There is no physical container for you to find. Your visit here will be a fact gathering trip to answer the questions below.
There is plenty of parking at GZ.
For many years, the region around Prim, AR has been known for its large round sandstone boulders, commonly ranging in diameter from 1 foot to 4 feet. These boulders are not restricted to the Prim, Cleburne County area. They have also been seen in situ in Alum Cove Natural Bridge and vicinity in southern Newton County to the west and in the Peacock Road Quarry in northern White County to the east.
Local farmers and ranchers knew about the spherical boulders in the Prim vicinity long before they were brought to the attention of geologists, because they were a nuisance to normal agricultural practices. Many of the boulders were moved from their original locations to the edges of fields or into piles and distributed along driveways, home entrances, or into front yards as decorative items.
Previous Work
In 1964 Tom Freeman, Arkansas Geological Commission (AGC) staff geologist, was the first scientist to write about the Prim boulders in an AGC Open-file report. His report gave documentation as to the land owners and the general distribution of the Prim boulders. He speculated that the boulders originated as iron-cemented concretions because he saw iron oxide banding on the exterior of many of the stones.
Worldwide, spherical boulders are not unique. They are well documented in geological literature and on the Internet. McBride, Picard, and Milliken (2003) studied the formation of large spherical sandstone concretions in Cretaceous deposits of Wyoming and Utah, They concluded that calcite cement was locally derived, most likely from original carbonate shell material contained within individual sandstone units. Fossil Beach on Kodiak Island, Alaska, is known for its spherical boulders, which formed in siltstone as calc cement grew radially around fossils. Near Tafraoute in Morocco, spherical boulders as large as small houses are present along the roadside. Researchers have attributed their formation to calcite cement derived from adjacent units rich in carbonates shell fragments. All of these documented have one thing in common: the boulders did not form from movement of water, wave action, or abrasion due to rolling down a slope. The sphericity is due to chemical, not mechanical, processes; carbonate formation internally is later expressed as an external spherical shape when released from the host rock by weathering.
Geology
The rock units in and around Prim, Arkansas are part of the Bloyd/Hale Formations of Morrowan age in the Pennsylvannia System. The units of sandstone and shale of the Bloyd/Hale were deposited in a delta-like environment some 300 million years ago. These sediments were deposited on a relatively flat plain that sloped gently to the south. Rivers built deltas out into the shallow ocean and deposited their sediments in a system of different deposits, typical of what is now observed at the mouth of the present day Mississippi River. The sediments were later buried by overlying materials and eventually compressed and cemented to become the rock units now observed.
The southernmost known occurrence in the Prim area is along Sugar Camp Creek. Here numerous boulders occur within alluvial fans or they are in the process of weathering from the outcrop. The boulders range from 1-7 feet in diameter.
On the eastern side of Prim, in Murphy Hollow there was a boulder approximately 12 feet in diameter. It is free from its host rock, in the creek bed, and was the only one seen.
On the western side of the Prim area, one-half of a spherical boulder was noted below a waterfall. This boulder was 1.5 to 2 feet in diameter and the only one found. The original location of this boulder is not known, but it may be upstream above the cliff face to the east.
Origin of Spherical Boulders
The origin of the spherical boulders in north-central Arkansas is related to several geological processes and a vast amount of geological time. When viewed in outcrop, the boulders are seen in every state of release from the host sandstone: from spherical masses embedded in the host rock, to accumulations of residual stones down slope from the host interval, and even to those that have reached creek level and been moved downstream by high water flow.
The process of formation probably began with the solution of calcite-bearing materials in the sedimentary units sometime shortly after deposition and compaction, and the nucleation and growth of the dissolved calcite in restricted sandstone beds. The calcite originated when associated carbonate animal remains (shells) were dissolved as the seawater was flushed from pore space by fresh water (McBride, Picard, and Milliken, 2003). No deformed spherical boulders were observed, so the sediments must have already been compacted before cementation began. These beds are usually 3-6 feet in thickness and are typically underlain and overlain by high-energy sand deposits displaying cross-bedding features in the vicinity of Prim.
After the initial growth of the carbonate cementation, the movement of groundwater carrying dissolved iron through the host rock started an additional cementation that resulted in some iron-oxide bands and often limonite-goethite case hardening of many of the boulders. The leaching of the carbonate and formation of case-hardened iron oxide banding are due to natural processes during weathering and erosion of the enclosing sandstone unit. As groundwater in the zone of oxidation slowly moved through the host rock, it encountered differing chemical and pH conditions associated with the carbonate-cemented boulders, when compared with the host rock. The iron oxide was deposited and the carbonate removed during this chemical exchange, leaving behind the boulders we see formed in natural outcrops.
Erosion of the host sandstone slowly removes the rock face on outcrop, and the cemented boulders are left as positive features. They survive the loss of host rock due to the superior cementation, and when released, they move down slope from the outcrop face, finally reaching creek level. This type of gravity-driven system of sown slope movement is generally more active in drier climates than presently exist in Arkansas. Greater vegetation cover holds underlying soil and loose material in place, slowing the run off of water. It is probable that many of the boulders reached their present positions some 10 thousand years ago when much of Arkansas experienced a drier climate than today.
The spherical boulders found in AR are not uncommon, as these types of concretions are known worldwide, but they are unique in size and local abundance. There appears to be as many as a thousand or more spherical boulders in the area near Prim and a short distance to the south into Cleburne County.
References:
Braden, A.K., 2005, Generalized stratigraphic chart for north-central Arkansas, Arkansas Geological Commission, Open-file Report
Freeman, Tom, 1964, Source and origin of spheroidal boulders, eastern Boston Mountain are, Arkansas, Arkansas Geological Commission, Open-file Report, 12p.
Hanson, William D. and Howard, J. Michael, Spherical Boulders In North-Central Arkansas, Arkansas Geological Commission, Open-file Report, p1-16.
McBride, E.F., Picard, M.D., and Milliken, K.L., 2003, Calcite-cemented concretions in Creataceous sandstone, Wyoming and Utah, U.S.A., Journal of Sedimentary Research, Vol.73, No.3, p462-483.
To demonstrate the educational value of this cache, please answer the following questions. Submit the answers to the following email: Ltexercise@yahoo.com Do NOT put your answers with your log. Anyone who logs the cache without answering the questions within one week of logging will have their logs deleted per Geocaching.com guidelines. Photos are optional.
****** Please include your Geocaching name and the number of people in your group with your email and answers ********
1. According to Tom Freeman in 1964 what did he speculate that the boulders originated from?
2. 2. Name 2 other counties in Arkansas, besides Cleburne, that spherical boulders have been found?
3. 3. The sphericity is due to chemical or mechanical processes?
4. Using your tape measure 11” above the ground, what is the circumference of the boulder at GZ?
5. 5. The house South of the post office has how many visible boulders lining their driveway?
Congratulations to Night_Hiker for FTF!
Additional Hints
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