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GEOLOGY OF THE POST ROCK LIMESTONE
Kansas is divided into 11 physiographic regions. The Smoky Hills region covers an area encompassing most of 23 counties in the north-central part of the state. Rush County is located in the southwestern portion of that region. Outcrops of Cretaceous age rocks are prominent in the region.
According to the Kansas Geologic Timetable, the Cretaceous Period was the interval of time from 142 to 65 million years ago at the end of the Mesozoic era. During this time, most of the western half of Kansas was covered by an inland seaway. As this seaway receded, it left behind deposits that today compose three principal rock outcrops, Dakota sandstone, Greenhorn limestone, and Niobrara chalk.
The lowermost of these, the Dakota sandstones, outcropping more to the east, are sediments and beach sands from the shorelines of the Cretaceous seaway. The uppermost layer, the Niobrara Chalk, extends in a vein westward from the western range of the Smoky Hills. These chalk deposits were in the deep parts of the Cretaceous Sea and are responsible for the unusual formations of Castle Rock and Monument Rocks in Gove County. It is also known for the discovery of fossils of swimming reptiles. The famous Fish-within-a-Fish fossil (Xiphactinus with Gillicus) on display at Sternberg Museum of Natural History is Hays was discovered in this region along with several plesiosaurs and mosasaurs that inhabited the water while dinosaurs walked the earth.
Situated between these layers is the Greenhorn Limestone, composed of alternating thin layers of chalky limestone and gray shale. The bed, approximately 100 feet thick, is further subdivided into four members. Lincoln limestone, the lowermost, is from 20 to 25 feet thick; Heartland shale, 30 to 35 feet thick; Jetmore chalk, 20 to 22 feet; and Pfeifer shale, 20 to 22 feet thick. At the top of the Pfeifer member is a thin bed of limestone called Fencepost limestone.
Eight to twelve inches thick, the Fencepost limestone layer often lies very near the surface (generally no more than three feet) and outcrops abundantly through the central part of the Smoky Hills region. It is this layer for which “Post Rock Country” gained its name. The fencepost layer is very uniform in thickness and free of joints or linear fractures. When first uncovered, the fencepost layer is soft, but after exposure to the air for a prolonged period, the limestone hardens. Since there were few trees in this region to build fences and buildings, settlers were forced to discover alternate materials. They soon discovered that this limestone bed suited their needs.
Limestone posts were also quarried from other layers within the Greenhorn limestone. In Cloud County, posts were quarried from the Shellrock layer about 20 feet in elevation below the Post Rock bed. The Shellrock bed is similar in thickness to the Post Rock, but is greyish-white in color and less consistent. There usefulness was limited, however, as they did not exhibit the strength or resistence to elements as posts from the Post Rock bed. In Ellis County, at the northwestern edge of Post Rock Country, posts were quarried from the yellow Fort Hays chalk. Since the chalk is softer than the Post Rock layer, the posts were considerably wider and crumbled when exposed to the weather.
A unique geological feature of Fencepost limestone is its characteristic brown streak (see photo), somewhat uncommon in limestone formations. Located near the center of the bed, the streak is a form of limonite (HFeO2), or more commonly, rust. No exact determination has been made as to the cause of the brown streak, but geologists have discovered a great presence of fossilized shells and driftwood within the brown streak. In Land of the Post Rock, Muilenburg and Swineford explain the source of the iron streak as being “…derived from alternation of finely disseminated pyrite and then redistributed during a process of recrystallizing and cementing of the limestone, followed by oxidation when the limestone is exposed to the air.” When iron rich water from swampy areas was washed up it carried along tree limbs, shells, and other debris depositing them on the shore.
In addition, they have discovered a greater presence of the driftwood in northern areas than in the southern parts of the Post Rock region. The presence of limonite, therefore, may be directly in proportion to the presence of fossil wood. Thus, the position and coloration of the brown streak may also help to pinpoint the locale of fencepost limestone.
To claim this earchcache as a find, you will need to email me the answers to the two questions that follow and you must take a picture of yourself in front of the evergreen tree just to the left of the outcropping.
The questions are as follows:
1. Measure the thickness of the limestone outcropping just to the right of the small evergreen tree.
2. Using an average density of 140lb/cu ft, if a settler had wanted to make a square fence post 7 feet long out of this outcrop, what would it have weighed?
Do not post your answers in your log. You must visit this site and post a photo of yourself here to get credit.
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