The posted coordinates are for an interesting rock on Scout Island, located in Greers Ferry Lake. A boat will be required to reach this island and visit this EarthCache.
REQUIRED INFORMATION
In order to gain credit for completing this EarthCache, please send me answers to the following. Answers may be sent through the Message Center (preferred), using the link found above below the cache name, OR by email through my profile page. DO NOT post your answers in your log. Found It logs that are not supported by the required information within 7 days of initial log posting will be deleted, as I must have proof that you successfully completed the requirements.
It is highly recommended that you read the description following this section, as it contains some of the answers to the required information and will also provide you with an explanation and better understanding of what you will see at the posted coordinates.
1) Viewing the rock at the posted coordinates, with the lake at your back, what shape does the rock appear to have? (HINT: Native American?)
2) There are several interesting patterns in the rock. Some are triangular. How many triangles do you count in the rock?
3) In your own words, how were these patterns formed?
4) Name at least 3 iron-bearing minerals that are found in Hartshorne sandstone.
5) OPTIONAL (I am just interested!): What was you first impression of this rock? That is, did it appeared to be natural, or man-made?
INTRODUCTION
At the posted coordinates, you will find an interesting rock. Look at the pattern in the rock, which forms rectangles and triangles. The pattern you see is called liesegang banding and is similar to that found in the Carpet Rocks in Petit Jean State Park (see GC37BY2). In fact, it is present in many of the sandstones throughout Arkansas so it is not unique to any one area of the state. Anywhere there is sandstone, you will probably find liesegang banding or boxwork. This phenomenon occurs in rocks (sandstone) as they are exposed to weathering, rain, and/or groundwater.
GEOLOGIC HISTORY
The sandstones found in Arkansas formed during the Pennsylvanian Period of geologic time around 315 million years ago. During the Pennsylvanian Period, two types of sandstone developed. One was caused by the buildup of sand, silt and clay from ancient river systems. Periodically, the river system areas were covered by a deepening ocean. Swamplands populated by ferns and trees unlike any today developed around bays and inlets. Sea level fluctuated during this time and as sediment subsided in the basin, sea water spread into the area covering the bays and inlets burying the swamplands. Several cycles of deposition created layers of clay and sand which contained thin layers of plant material. These layers became buried and compressed by overlying deposits to form shales and sandstones with an occasional thin coal bed. These layers of rock are known as the Atoka Formation.
Eventually the ocean retreated west and the in-filled basin became exposed. A large river system flowing east to west developed in the low-lying area. Swamplands developed on adjacent flood plain deposits and later became buried when the river meandered and changed its course. This river deposited sandy sediment that became buried and compressed to form sandstone sequences that in some areas contain plant fragments.
One interesting feature that has formed from weathering of the sandstone is called liesegang banding, which results in box-shaped and triangular patterns in the sandstone. Iron exists as the minerals siderite, magnetite, hematite and some clay minerals that are present in the sandstone. At some point in geologic history water fills the pore spaces of the rock formation and comes into contact with these minerals made up of iron. This causes the iron to go into solution. If the rock becomes exposed to air then oxygen is added to the solution and will cause the iron to oxidize and precipitate out along exposed joints in the rock formation. Sometimes color bands result from the different oxidation states of iron. These bands are also referred to as Liesegang banding or box-work by the scientific community. Often the joints form interesting triangles, as in the rock found at the posted coordinates.