Logging tasks:
1. Do the columns or pillars seem to be of the same size? Why is this?
2. What fossils did you see during your visit?
3. Explain in your own words if you think the pillars will continue to grow or if they will disappear over time.
4. Post a picture of yourself (face not required) at the 7 pillars.
This EarthCache will bring you to the Seven Pillars of the Mississinewa. Mississinewa means "River of Great Stones" in the Miami Nation language. The river is a tributary of the Wabash River.
From the south side of the river at the posted coordinates, you can see the 7 Pillars from a wide vantage point at the river level. On the day of my visit I was able to wade across in knee deep level to get a closer view of the limestone pillars. The pillars are mainly composed of Liston Creek Limestone and the base is a silty dolomite. The overhanging ledges that make up the "roof" of the pillars is made entirely of Liston Creek Limestone.
The beds show distinct ripple marks which would have been formed by waves or wave-driven currents of the water. The beds themselves are mostly 2-5 cm thick with a flaggy, wavy look. The layers are irregularly shaped and have nodules of chert. Fossils are visible here are from swimming organisms that would have burrowed in the mud or were fixed to the ancent sea floor. Crinoids, trilobites, gastropods and nautiloids are typical.
So why are the pillars here? Basic answer is weathering. The weathering and erosion patterns that show in the rocks here occurs at different scales. On a smale scale, the rocks slake off the outcrops in thin, sharp edges along the bedding planes and fractures where the chert and limestone meet. The process seems to happen faster in certain areas of the outcrops, which makes ledges, overhangs, and alcoves that parallel the bedding planes. On a larger scale, the rocks are cut by 60 and 120 degree angles. Also these joints, the weathering is much worse than in the sold rock between them. Some of the weathering is from solution, which makes the joints visible from the top much bigger. These joints allows the water (moisture) to penetrate down into the rock, which will also allow freeze/thaw cycles to further break apart the rocks. When the rock is removed from the joints, they will leave intervening colums of rock standing as "pillars".
So why at this location? The pillars are located on a cut bank. The flooding waters will scour away the loose soil and rock from the face. These will keep the bedrock exposed. As time goes on, the cutbank slowly retreats to the north. The south bank is an alluvial plain that is built on a large point bar. During low flow, you will see the active part of the bar just offshore in the river channel. At one point, the river channel was at the base of the bluff. Over time, the flooplain as enlarged as the point bar moves north keeping pace with the cut bank retreating on the opposite side. As this continues, a large meander loop and will cut off and abandon during large floods. As it keeps moving during future floods, the river will no longer erode the pillars and the pillars will turn into slopes as they wear down and become covered by soil.
Logging tasks:
1. Do the columns or pillars seem to be of the same size? Why is this?
2. What fossils did you see during your visit?
3. Explain in your own words if you think the pillars will continue to grow or if they will disappear over time.
4. Post a picture of yourself (face not required) at the 7 pillars.
Sources: Roadside Geology of Indiana, GSA Field Guide 51