Flint Hills Geology

Though the Flint Hills have been roughly 4.5 billion years in the making, an examination of the surficial geology (surface rocks and features), reveals that their formation occurred primarily during the Permian Period some 290 to 250 million years ago (Buchanan, 1984). During the Permian, the area that is now known as the Flint Hills underwent periodic cycles of transgression and regression of seawaters (each cycle is termed a cyclothem). The transgression phases involved water encroaching onto the land, resulting in large expanses of flooded areas.The regression phases involved the water receding to lower levels, resulting in more shallow inland seas, and at times,dry, open lands (Merriam, 1963). Rocks that were laid down during this time exhibit what geologistscall "layer-cake stratigraphy" and are composed primarily of alternating layers of shales and limestones (Muilenberg, 1959), interbedded in places with sandstones (Buchanan, 1986). Generally speaking, it is believed that limestone depositions accumulated in areas of deep-sea waters (Buchanan, 1986). Shales accumulated in areas that were slightly above sea level or in near shore marine environments. Sandstones accumulated in areas of non-marine deposition, such as rivers, lakes or land (Muilenberg, 1959).

With respect to the Flint Hills region, the limestone layers that were deposited in the Permian Period contained an abundance of a mineral called chert, or flint (also regarded as a rock), from which the region drew its name. According to Merriam (1963), the overall composition of some limestone layers contains as much as one quarter to one third chert. Chemically, chert is a silicate, composed of silicon dioxide, and is one of the many varieties of quartz. The origin of chert is not fully understood, however, one hypothesis suggests that as the chemical composition of the Permian seas changed, sponges living in the open waters began producing spicules of silica that was extracted from the seawater (Muilenberg, 1963).

Well below the surface of the Permian hills lies the basement rock layers (Precambrian through Mississippian deposition).The east-west dipping of the basement rocks is responsible for much of the surface topography that is observed throughout the Flint Hills today. As previously mentioned, chert is extremely resistant to erosion. At the same time, the embedded layers of shale are easily eroded by both wind and water. This combination of erosion-resistant limestones and easily eroded shales, overlying a basement rock that is tilted toward the west, results in the distinct surface topography foundin the Flint Hills. This topography is characterized by steep, eastward facing slopes where the shale has eroded and the limestone layers remain, as well as much more gently sloping westward facing slopes (Buchanan, 1984). These slopes can easily be seen in the photograph below.