Flint Hills:
A Physiographic Region of Kansas

By Andy Holt
Field Geomorphology
Fall 2007


Flint Hills Along Highway 150 Near Highway 50
Image provided by Terry G. Holt

Table of Contents

Introduction
Geomorphology
Geologic Timeline
Climate
Land Use
Methodology
Conclusions
References

Introduction

The Flint Hills is a physiographic region of Kansas known for its chert rich limestone which makes it highly resistant to weathering. This region spans across twenty counties from Marshall and Washington counties in the north and continues down south into Oklahoma where it is known as the Osage Hills (Figure 1). The width of the Flint Hills varies from 50-160 km (National Park Service). The topographic area has a range in elevation from 500 m to 350 m with local relief varying as much as 100 m. Perennial rivers such as the Walnut, Cottonwood, and Verdigris have incised down into the local bedrock. The bedrock of the region, mainly limestone and shale beds, were initially laid down during the Permian Period approximately 250-290 million years ago (Aber, 3). At that time, Kansas was under a shallow inland seaway known as the Permian Sea. Due to the interbedding of the limestones and shales, the area has weathered to reveal what is characterized as "layer-cake" or "bench and slope" topography. A large portion of the area today is used as ranch land and it is also home of the Tallgrass Prairie National Preserve. The Tallgrass Prairie National Preserve is home to North America's largest native tallgrass prairie.



Figure 1: The Physiographic Regions of Kansas
Image taken from Kansas Physiographic Regions: James S. Aber
Modified by Andy Holt

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Geomorphology

The Flint Hills are characterized by rolling hills topped with weather resistant chert. The chert is believed to have come from the local region. Much of the chert is exposed through weathering of the cherty limestone beds. Some exotic quartzite pebbles have been found in the chert gravels atop hills. These pebbles are believed to have been transported from the High Plains physiographic region out in western Kansas and/or the Rocky Mountains of Colorado (Aber, 3 and 4). The hills are usually found topped with thin soils with concave slopes. The interbedding of the limestones and shales provides for a "bench and slope" topography. The limestone is resistant to weathering and forms a cap. Eventually, sections of the limestone are weathered away to expose the underlying shale. The shale erodes relatively quickly, and exposes the next limestone layer. In some areas evaporates, such as salt or gypsum, can be found. These formations were created as the Permian Sea dried up and precipitated out the minerals it had in solution.

A regional dip from east to west is exhibited by the bedrock. The degree of dip varies from 4 m/km to 8 m/km. Other directional dips to the north, south, and east can be found throughout the area up to rates of 11-15 m/km (Aber, 3). This is representative of the more pronounced hills in the east which slowly give way to a rolling plateau. The eastern facing slopes are found to be at a steeper gradient than the gentler western slopes. Wind plays a role in shaping the landscape, however, the primary method of erosion are the rivers and streams of the area. The Cottonwood and Verdigris among others have incised down into the bedrock. Most channels are steep and have incised down into the limestone with an almost box-like in appearance (Figure 2). The image below demonstrates how the streams and rivers of the Flint Hills follow a box-like pattern. The waterways follow naturally occurring cracks in the bedrock which were formed through folding of the underlying strata (Aber et al.). In areas where shale is the top-most exposed layer, the rivers tend to be more broad.



Figure 2: False Color Landsat Image of the Central Flint Hills
Image taken from Applications of Landsat Imagery in the Great Plains
Modified by Andy Holt

The geomorphic feature known as karst is prevalent throughout the region. Karst occurs when groundwater dissolves limestone beds and creates caves, sinkholes, and springs. Fractures in the bedrock are advantageous to the development of caves, sinkholes, and springs. In areas around Butler County, karst has produced numerous sinkholes; in many places it is common to find 10 for every quarter section (Aber, 4). These sinkholes are readily found in the Ft. Riley Limestone Member. Three caves in central Butler County have been named and mapped. All three are interconnected by underground waterways. The caves are all joint controlled, but Spring Cave demonstrates this to a higher degree than both Smith and Windmill Caves. The groundwater from this network of caves and sinkholes feeds a perennial spring which lies to the west of the formation (Aber, 4).

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Geologic Timeline

Transgressive and regressive cycles of the Permian Sea created the bedrock stratigraphy found throughout the Flint Hills. As marine invertebrates and vertebrates in the Permian Sea died, their skeletal remains fell to the bottom of the sea. They were eventually compacted and lithified to form the limestone beds found today. The shales of the area formed from compacted and lithified clay and silt sized particles which had washed out to sea from the lowland coast. The repetition of limestone, shale, limestone, shale, etc. make up structures called cyclothems. In the Flint Hills, the larger cyclothems range from 10-30 m thick. Examples of such cyclothems are the Blue Springs/Barneston and the Speiser/Wreford (Aber, 3). Smaller cyclothems are 8-12 m in thickness with examples being Easly Creek/Crouse and Eskridge/Beattie (Figure 3). These formations were being laid down on top of an active uplift known as the Nemaha Uplift. According to Gao et al. the Nemaha Uplift has been an ongoing process over the past 600 million years. Dr. James Aber believes that the uplift was probably formed by rifting of the continental crust during the late Proterozoic and reactivited during Ouachita Orogeny in the late Paleozoic. The strata of the Flint Hills have few inconsistencies. The most noticeable inconsistency is the thinning of the beds which directly overlay the Nemaha Uplift. This lends credence to the theory that the limestone and shale beds were being laid down as the uplift was occurring. Eventually the Permian Sea dried up completely. The region remained dry until mid-Jurassic when another sea began to form. That sea developed into the Great Inland Seaway of the Cretaceous Period. Once again the waters receded. During the ice ages the furthest glacier advancement stopped along the northeastern edge of the region before withdrawing. Only within the past 20,000 years has the Flint Hills flora changed from a pine savanna to a tallgrass prairie (Aber).

Figure 3: Bedrock Stratigraphy of the Flint Hills from the Permian Period
Image taken from Tallgrass Prairie National Preserve
Modified by Andy Holt

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Climate

The Flint Hills is classified as a sub-humid continental climate (National Park Trust). The average temperature year round is 13° C (55° F) and a range of 76.9-92.3 cm (30-36 in) of annual precipitation. This allows for a growing season of approximately 180 days. The region experiences flash flooding and other severe weather such as tornadoes.

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Land Use

A small portion of the Flint Hills is a part of the Tallgrass Prairie National Preserve. The preserve is the largest native tallgrass prairie in North America. Native grasses include switch grass, Indian grass, along with big and little bluestem grass (KGS). The primary land use in the Flint Hills is ranching. To this end farmers have built numerous watersheds to help slow the rate of erosion and to provide water for livestock (Figure 4). Farming is present in the region, but to a much lesser extent than ranching. The relative lack of farming is due to the abundance of chert nodules and little topsoil. Topsoil is thicker near and around waterways thanks to fluvial deposits. Flash flooding is a common occurrence. This is to be expected when severe thunderstorms rain on more or less impermeable soils and bedrock . To combat flooding, reservoirs were constructed at Council Grove, El Dorado, and Marion (Aber, 2). These efforts have been somewhat successful. The success has been mitigated by the fact that many waterways in the Flint Hills have no such preventative measures in place. Major floods occurred in the region in October 1985, July 1993, May 1995, April 1997, November 1998, and March 2004. Secondary uses of the reservoirs include recreation and continuous water supplies.

Figure 4: Watershed Along Highway 150 West of Highway 50
Image taken by Terry G. Holt
Modified by Andy Holt

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Methodology

The mission of this course was to study the physiographic regions of Kansas through field work and to take pictures of the various regions using small format aerial photography (SFAP). This was done using kite aerial photography (KAP) in some locations and blimp aerial photography (BAP) in other locations. KAP is done using a hexagonal kite with support bars. The kite was released into the air with the line let out. Further down the string a camera in a remote controlled rig is attached to the kite string and allowed to ascend (Figure 5). Various cameras such as the Cannon Elph, Rebel, and S70 can be used in this manner. A two person team, a spotter and the remote control operator, work together to take pictures with a 360° view. BAP is done in a similar manner with the same camera models, but in this case the camera rig is mounted directly onto the blimp. BAP is only done with virtually no wind. KAP can be conducted in somewhat higher wind speeds, but the wind cannot be gusting. Due to inclement weather, KAP was unavailable while studying the Flint Hills. Pictures of the Flint Hills were taken and provided by Terry G. Holt using an Olympus EVOLT Digital SLR camera.

Figure 5: Camera and Rig Used in KAP
Image taken from Kite Aerial Photography: Cameras and Rigs: James S. and Susan W. Aber
Modified by Andy Holt

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Conclusions

The Flint Hills is a physiographic region of Kansas spread out over twenty counties and south into Oklahoma. It is characterized by cherty hill tops, limestones interbedded with shales which formed during the Permian, and host to karst formations. A regional dip can be found declining to the west which the waterways following joints in the underlying strata. The Flint Hills is classified as a sub-humid continental climate with the potential for flash floods. Reservoirs have been built to reduce the risk of flooding, but many of the smaller rivers and streams flood regularly. The region is home to the Tallgrass Prairie National Preserve and is primarily used as ranch land.


Flint Hills Along Highway 150 Near Highway 50
Image provided by Terry G. Holt

 

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References

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This webpage was created for public use as a term project for ES546 on Nov. 15, 2007. For more information contact aholt@emporia.edu