Osage Cuesta Geology Today
Glossary of Terms
Kansas is divided into 13 distinct geographic regions. Each of these regions shows distinct landforms and topography. Each of these regions is also a direct reflection on the underlying geology. The rocks that make up these regions are oldest and lowest in elevation to the southeast, growing progressively higher and younger to the west.
The Osage Cuestas region of Kansas covers most of the southeastern portion of the state. The foundation rocks of this region are among the oldest exposed in Kansas. The Osage Cuestas are typified by rolling hills and low ridges that are steep on one side and gently sloping on the other. These landforms are known as cuestas.
|A map showing the physiographic regions of Kansas. Image by J.S. Aber.|
|Typical Osage Cuesta topography. Used with the consent of the Kansas Geological Survey.|
The Osage Cuesta region provides Kansans with a variety of natural resources. Agriculturally the region is primarily pastureland. Water is abundant in the region and supports some crop cultivation, primarily animal feedstock. Much of Kansas mineral wealth lies beneath the Osage Cuestas. A once thriving coal industry flourished in the southern part of the region. Today, there is a resurgence of petroleum development from the organic rich Pennsylvanian strata. Kansas is also a major producer of limestone aggregate and cement, both of which are mined from the Pennsylvanian rocks of southeast Kansas.
|>||Another view of Osage Cuesta topography taken using Kite Aerial Photography. Image by J. S. Aber.|
The Osage Cuestas also provide an abundance of recreational opportunities. Game and wildlife thrive in the hardwood fence lines and pastures. The water resources also make this region rich as a recreational fishery. Much of the water supply of eastern Kansas is contained in the many reservoirs of the Osage Cuestas.
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Geologically, the Osage Cuestas is composed of marine sediments deposited approximately 300 million years ago during an era known as the Pennsylvanian Period. Today these sediments form the limestones and shales that are seen in road cuts and quarries throughout the southeastern portion of the state.
During the Pennsylvanian, Kansas was covered by a shallow sea that stretched from what is now the western portion of the Rocky Mountains to the modern Atlantic seaboard. This sea was flat and shallow, with many marshy emergent areas and islands. It was in these marshes that the vast coal resources that stretch from Utah to West Virginia were formed.
These shallow seas were subjected to frequent changes in sea level, most likely due to fluctuating global glaciation common throughout the period. As glaciers grew they would draw water from the sea and store it as ice. Because the sea was so shallow the sea level changes could move shorelines a great distance in a short time. As water levels fluctuated different types of rock were deposited in a steady sequential progression.
Today all that remains of these shallow seas is the rock they left behind. This rock manifests itself in the landscape we see everyday. The Osage Cuestas are a direct reflection of the marine history of southeast Kansas.
|Different depositional environments as sea level rises and falls.|
Because the water levels of the Pennsylvanian sea fluctuated with regularity, a predictable series of rocks were deposited. These rocks reflected the varying depths of the sea at the time they were laid down. These series repeated again and again for millions of years until thousands of feet of sediments were piled under what would become Kansas. This series of predictable sedimentary facies is known as cyclothems. Cyclothems are known across the United States in strata deposited during the Pennsylvanian seas. Sometimes, when sea level retreated far enough for a marsh to develop, layers of vegetation would accumulate and form coal deposits. Coal is a well know product of cyclothem depositional environments.
|Cyclothems appear as repeating patterns of similar geologic units in outcrops.|
Over time, these sediments were buried by rocks of different environments. Seas came and went, the Rocky Mountains arose and buried Kansas in a wash of sand and gravel. During this time, forces deep within the Earth warped and contorted the underlying bedrock and sediments until all of the buried Pennsylvanian formations sloped gradually to the west and northwest. Since the time of the Pennsylvanian seas, erosion (driven by the drainage of surface waters to the Mississippi River basin) has periodically stripped away the cover from Kansas from east to west.
|After the strata is tilted, the differential resistance of the limestone forms escarpments above the surrounding weathered shales.|
This erosion is the final piece in the puzzle of landforms. Different materials weather away at different rates. As the gently dipping Pennsylvanian aged rocks were exposed, the limestones deposited in deep waters proved more resistant to weathering then the shales and sandstones of shallow deposition.
Wherever the limestone outcropped, it resisted erosion while the shales were stripped away. Ultimately this left the limestone standing higher then the surrounding layers. Because of the gentle slope, the limestone ridges tended to be steep on the up gradient side forming an escarpment. On the down gradient side of the ridge, the topography was more gentle along the top of the limestone beds. This alternating steep and gentle slopes is what defines cuesta topography.
|An example of a limestone – shale contact seen in a road cut.|
Cuesta development, like nearly all landforms, is predominately a function of differential weathering. The different lithologies, dipping strata, and joints and fractures all combine to create the topography of southeast Kansas. The abundant water in the region is the driving force behind the stripping away of thousands of feet of sediment. As water supplies diminish to the west, elevations increase and younger and younger rocks cover the surface. The oldest rocks exposed in Kansas are found to the southeast of the Osage Cuesta region, where Kansas surface waters are most prevalent and have been able to erode more cover.
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The topography seen in southeastern Kansas today has a direct relationship to the geology of the underlying rocks. These rocks are the persistent remains of a sea that covered most of North America over 300 million years ago. The Osage cuestas generally run in low ridges the strike from north to south. The escarpments generally face the east. These escarpments are frequently incised by stream channels or in many instances have been eroded to the point that only isolated knolls are visible. The topography has its highest relief in the west along the border with the Flint Hills region and is flattest in the southeast as it nears the Cherokee Lowlands.
Today all that remains of these shallow seas is the rock they left behind. This rock manifests itself in the landscape we see everyday. The Osage Cuestas are a direct reflection of the marine history of southeast Kansas. Evidence of the marine history can be found in the many fossils that are found in the limestone and shale outcrops across the Osage Cuestas region.
All of the landforms evident across Kansas are a testament to the hundreds of millions of years of geologic forces that shaped North America. The constant cycle of erosion and deposition is still at work today. In some areas sand and gravel carried from the Rocky Mountains are being piled up in Kansas river valleys, while in others, erosion continues to expose older and deeper rocks. Over geologic time, the Osage Cuestas will gradually migrate westward as Pennsylvanian strata are exposed to the west and stripped away from the east.
|An example of a remnant of the formations that formed the Osage Cuestas in Neosho County.|
|A diagram of the above structure. The durable limestone protects the underlying shale from erosion.|
Need a definition? Check the Glossary of terms used in this report.
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Geofacts – Osage Cuestas Rocks and Minerals, World Wide Web homepage URL:
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Buchanan, Rex (Editor), 1984. Kansas Geology, University Press of Kansas. 208 pp.
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Wagner, Holly C., 1964. Pennsylvanian Megacyclothems of Wilson County, Kansas, and Speculation Concerning Their Depositional Environment, U. S. Geological Survey Bulletin 169, p. 564-591