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Geologists in the area say that one of the best-known anticlines in the whole district is that in Backbone Ridge, between Bonanza and Hackett, which extends from the State line eastward for 7 or 8 miles. The coal-bearing beds which outcrop on both sides of this ridge are steeply tilted and in its axis rocks lying far below the coal horizon are exposed. This anticline is an unsymmetrical fold of the Ouachita Mountain type and represents a thrusting of the rocks from the south over those on the north side of the axis. The strata are broken, and a well-marked thrust fault has been traced along the axis for several miles.
The superposition of beds, of sandstone, of shale, of limestone, and of coal, one upon the other, and the horizontal continuity of these rocks over great distances, cannot have escaped the notice of even the most careless observer. The great beds of mountain capping sandstones so prominently exhibited in the escarpments of Petit Jean mountains, the hills at Ozark, at Van Buren, near Enterprise in Sebastian county, and at innumerable other places are but remnants of such layers once connected in great overspreading sheets. These layers were formed under different conditions: the limestones in ocean depths, the sandstones in the more rapid currents of shallower waters, the shales and coals probably in the shallow and well- nigh stagnant waters of endless lagoons and marshes, rich in a tropical vegetation. All these beds, however, were deposits in the form of layers, or great sheets, one upon the other like the leaves of a book, and "are known, in geology, under the familiar term "strata."
Another equally apparent fact, however, is that while the various rocks are arranged in successive layers or strata, these layers are often not horizontal but are tilted at an angle with the horizon, sometimes slight and sometimes nearly vertical. Such disturbance is beautifully exhibited along the crest line of Devil's Backbone ridge, south of Greenwood in Sebastian county. This flexing of the rocks leads one to a consideration of the subject of geologic dynamics. The tilted position of the strata is not the original one, but has resulted from a bending, crumpling and upheaval of the once horizontal beds, due in some cases to a shrinkage of the earth's crust, in others to local seismic or volcanic action.
In this case, we are looking at a thrust fault coming up from the south.
Superposition is a geological concept that is defined as the principle that in a group of stratified sedimentary rocks the lowest were the earliest to be deposited.
Looking at the north end of the west side of the road cut, you will see a large brown strata, a white strata, and a black strata. The brown and white strata are different types of limestone, while the black rock is shale.
It is suggested if you wish to climb one side of the rock cut for a better view, climb the east side.
To complete the EarthCache, complete the following items:
1. Take and post a picture of the west side of the roadcut. You may be in the picture if you want, or you can put your GPS, or sign, or whatever. Do what's comfortable. Make me smile.
Send the answers to these questions to Ashallond via email:
2. Referring to the concept of superposition, which layer was deposited first: the brown limestone, white limestone, or black shale?
3. Limestone and shale are deposited in different environments. Suggest how the different types of rocks could be formed in the same place.
The Earthcache is a rockcut on Arkansas State Highway 45 south of Bonanza. The old road around the east side of the mountain still is in place, and can be used for safe parking. If you wish to climb the eastern side of the cut for a better view or nearby caches, the south end is not as steep.
Additional Hints
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