GEOMORPHOLOGY OF
THE FLINT HILLS

EAST-CENTRAL KANSAS

James S. Aber
Earth Science Department
Emporia State University
Emporia, Kansas 66801

Table of Contents
Introduction Soils & unconsolidated sediments
Leon Gravel Exotic pebbles
Structural control of drainage Karst and caves

Introduction

The Great Plains of the central United States are often portrayed as a tectonically stable terrain of low relief, in which near-peneplain conditions prevail. This point of view is no longer considered valid, in light of abundant evidence for Quaternary neotectonism and pervasive erosion (Madole et al. 1991). In eastern Kansas, massive erosion of the landscape has taken place since Miocene time, rivers are entrenched, and valley patterns follow well-defined lineament systems. Contrary to popular belief, eastern Kansas has an active geomorphic regime dominated by stream erosion, which is strongly influenced by bedrock structure and neotectonism.

Alluvial deposits of chert gravel of presumed Neogene age are widespread and abundant on hill tops and high terraces throughout eastern Kansas. High-terrace chert gravels mostly are associated with modern stream valleys. In contrast, hill-top chert gravels in many places bear no obvious relationships to any river systems of today. Many, but not all, of these gravels contain quartzite and other exotic pebbles derived from the High Plains and/or Rocky Mountains to the west.

Soils and Unconsolidated Sediments

Soils series are useful as indicators for certain geomorphic processes and/or landforms. Some soils correspond to kinds of unconsolidated sediments that cover bedrock. Other soils are eroded with thinning of the surface layer; some are gullied or display local exposures of bedrock. Selected soils and corresponding unconsolidated sediments for Butler County are summarized below, based mainly on Penner et al. (1975).

Residual chert accumulation on the Ft. Riley Limestone near Rosalia, Butler County. The Florence soil is formed in such residual chert beds. Photo date 10/89, © J.S. Aber.
Olpe soil section in uplands south of Augusta, Kansas. This is a two-part soil--upper portion is dark brown wind-blown loess, lower portion is reddish brown chert-gravel alluvium. Photo date 10/89, © J.S. Aber.
Floodplain in the foreground rises onto a low terrace in the background, near Latham, Butler County. The car and hay bales are positioned along the edge of the terrace. Photo date 11/90, © J.S. Aber.
Tombstones have tilted due to heaving of the loess soil. Holderman Cemetery, Butler County. Photo date 6/90, © J.S. Aber.

Some of these same soils are also mapped in Chase County, along with other soils that are distinctive for unconsolidated sediments and/or surfaces of active erosion--Table 2. Some of these soils may be reliable geomorphic indicators. For example, Olpe and Smolan soils correspond to old alluvial deposits in upland positions. The Sogn soil indicates shallow or exposed limestone; Florence soil is associated with residual chert weathered from bedrock; and several soils are associated with low terraces or floodplains--Figures 12 and 13.

Table 2. Selected soils of Chase County and their sediment and landform situations. See soil survey for letter codes (Neill 1974).
Code Sediment/Landform
Ar, Iv Alluvial deposits on valley floors and floodplains. These surfaces flood frequently--Figure 12.
Ch, Ka Os, Ra, Rd, So Alluvial deposits on low terraces. These surfaces may flood occasionally--Figure 12.
Om Alluvial chert gravel on high terraces and hill tops. These surfaces are never flooded.
Sm Alluvial silt/clay on high terraces and hill tops. These surfaces are never flooded.
Fa, Fm, Tu Residual chert gravel weathered from bedrock. Located on hill tops and uplands--Figure 13. Associated with Wreford or Florence bedrock sources.

Leon Gravel

Upland chert gravel deposits are widespread in southern Butler and northern Cowley counties--Figure 14. These gravels are formally designated as the Leon Gravel after the city of Leon (Aber 1992). A stratotype is designated in NE¼ sec. 35, T27S, R5E--Figure 15. As defined, the Leon Gravel is equivalent to the Olpe or Olpe-Norge soil map units within the Walnut drainage basin. The Leon Gravel is usually 1-2 m thick and rests on bedrock. It can be informally separated in some places into upper and lower members on the basis of topographic position in relation to adjacent modern valleys.

The stratotype vicinity demonstrates typical geomorphic conditions for the Leon Gravel--Figure 15. High-terrace and hill-top deposits are preserved north of the Little Walnut valley at 6-12 m and 18-25 m respectively above the valley floor. In places, gravel is continuous between the high-terrace and hill-top positions. Hill-top gravels preserved south of the Little Walnut valley are situated on the local divide between Little Walnut and Hickory basins. Hill-top gravels gradually rise in the downstream direction relative to the modern Little Walnut valley. In the stratotype vicinity, highest gravels are 25 m above the nearby valley floor. Downstream on the divide between the Little Walnut and Walnut basins, hill-top gravels are up to 30 m above the valley floor.

High terrace along the south fork of the Cottonwood River at Bazaar, KS. The former school house and cemetery are situated on the terrace surface, which is underlain by 3 m (10 ft) of alluvial chert gravel (Olpe soil). Photo date 10/92, © J.S. Aber.

The Leon Gravel is thought to be more-or-less equivalent to Neogene chert gravels that are common in the Neosho, Verdigris, Fall, and Marais des Cygnes basins east of the Flint Hills. Such gravels are also common in the Cottonwood basin of Chase County. Hill-top gravel is assumed to be Neogene (late Miocene and/or Pliocene), and high-terrace deposits may be Pliocene and/or early Pleistocene. However, the true ages of the Leon Gravel and similar upland gravel deposits are uncertain.

Raster grid of upland chert gravel deposits (Olpe soil) in eastern Kansas, according to township-and-range grid. Each pixel represents one quarter section in area (0.25 mile², 0.65 km²); pixels are color coded according to elevation classes. Numbers indicate man-made reservoirs: 2 = Pomona, 3 = Melvern, 5 = Marion, 6 = John Redmond, 7 = El Dorado, 8 = Toronto, and 9 = Fall. Taken from Aber 1997.

Exotic Pebbles

Exotic pebbles of quartz and quartzite are present rarely in many, but not all, hill-top and high-terrace gravel deposits of the Flint Hills and eastward. These exotic pebbles were presumably derived from High Plains and/or Rocky Mountain sources. Most were probably reworked from Ogallala-type deposits, and some may be derived from Cretaceous strata. These sources are located in the Arkansas Lowland and Smoky Hills of central Kansas--Figure 1.

Exotic quartzite and quartz pebbles from upland chert gravel south of Douglass, Butler County. Exotics are especially abundant in this deposit of the ancient Walnut River. Photo date 1/91, © J.S. Aber.

In southwestern Marion County, the Goessel Plain marks the eastern edge of the Arkansas Lowland. This plain is formed by unconsolidated sediment, up to 15 m (50 feet) thick, that forms a thin cover over Wellington Shale. This sediment is mainly alluvial sand and gravel of arkosic composition, which was derived from the Rocky Mountains, and is of late Tertiary and/or early Pleistocene age.

The edge of the Goessel Plain marks the divide between the Cottonwood and Arkansas drainage systems. This edge is slowly retreating westward, due to steeper gradients and more aggressive erosion in headwaters of the Cottonwood system. At one time, the alluvial plain must have sloped gently eastward across the bedrock terrain of central and eastern Marion County--Figure 16. Subsequent stream erosion has removed nearly all of the alluvial plain in Marion County and has deeply dissected bedrock across the Flint Hills region.

Remnants of former alluvial gravel can be seen in upland fields of eastern Marion County. Scattered pebbles are resistant types--quartzite, chert and milky quartz, which bear glossy polish of wind abrasion. Some have ventifact shapes. The chert and milky quartz are derived from local bedrock, specifically cherty limestones. The quartzite pebbles were, however, transported from western sources in the High Plains and/or Rocky Mountains. Exotics are identical in type and color to those found in chert gravel (Olpe soil) to the east.

Quartzite pebbles on hill tops of eastern Marion County are all that remain of a once-extensive alluvial plain that reached from the Goessel vicinity eastward across what is now the Flint Hills. Chert gravel bearing exotic quartzite pebbles is also preserved in high positions of the Flint Hills in western Chase and eastern Butler counties at elevations up to 450 m. Quartzite pebbles in these areas are further evidence for alluvial sediment transportation across the Flint Hills during the Neogene.

It is likely that the ancestral Arkansas and Verdigris (Smoky Hill) rivers once flowed eastward from the central Kansas vicinity, across what is now the Flint Hills, and into the Neosho and Fall drainage basins (Aber 1997). During the Neogene, the Flint Hills did not exist as a prominent upland region; the Flint Hills emerged later, as stream erosion lowered terrains to the east and west. Because of their greater resistance to erosion, cherty limestone bedrock maintained high topographic relief in Flint Hills. The modern river system evolved via a series of stream captures and drainage diversions during the process of valley entrenchment.

Structural Control of Drainage

It is clear that bedrock structural features have exerted a strong influence on stream erosion patterns in the Flint Hills. Streams follow the troughs of synclines in several places--Figure 17. Joint sets have likewise influenced drainage development, as seen in the Walnut basin. Most eastern tributaries of the Walnut River flow downdip, parallel to joint set 1 (50-65°); examples include the Little Walnut River and its North Branch and portions of Rock Creek. The western tributaries of Walnut River follow valleys that are mostly parallel to joint set 2 (310-335°), including the West Branch Whitewater River. Joint set 3 (15-35°) relates to the main stem of the Walnut River northeast of Augusta, including the west arm of El Dorado Lake. These latter two trends correspond to major lineaments found across eastern Kansas--Figure 18.

The upper North Branch Verdigris River flows toward the NNW as does the South Fork Cottonwood, and headwater streams of the Fall drainage flow opposite to the SSE. These valleys follow the Verdigris lineament trend (Aber et al. 1997). The upper North Branch Verdigris turns abruptly toward the east in southeastern Chase County. This is one of the most prominent drainage anomalies in Kansas. At one time, the North Branch Verdigris River may have extended farther west, as the ancestral Verdigris, but the western portion was captured by headward erosion of the South Fork Cottonwood River.

Landsat multispectral scanner (MSS) image of central Flint Hills region in southern Chase, northwestern Greenwood, and northeastern Butler counties. Standard false-color composite, in which active vegetation appears pink and red. Vegetation is active within valleys, and Flint Hills uplands appear in green-orange colors in this autumn image from a drought year. Taken from Aber (1997). Click on the small image to see a full-sized (362 kb) version.

Upland chert gravels throughout eastern Kansas are preserved almost exclusively on the northern sides of W-E river valleys, and on the western sides of N-S valleys--Figure 14. The same patterns hold true for lower (= younger) terraces within these valleys, and modern rivers are cutting bedrock bluffs in many places on opposite valley sides. Only a few exceptions are known, mostly in connection with structural domes or basins developed over the buried Nemaha ridge in Butler and Chase counties. Valley asymmetry is the result of systematic river migration during downcutting, which may reflect long-term crustal warping downward to the south and east across much of eastern Kansas (Aber 1997).

View westward over Lake Kahola, KS. The south (left) side of the valley is marked by steep bedrock bluffs, which are forested. The north (right) margin of the lake is an old alluvial (chert gravel) terrace that gently slopes into the lake. Kite aerial photograph, date 7/99, © J.S. Aber.

This pattern is manifested both in the distribution of older upland gravel as well as by lower terraces and bedrock bluffs within modern valleys. Crustal tilting downward to the south andeeast is, thus, a long-term condition which is still continuing in eastern Kansas. Crustal tilting had the effect of increasing gradients for streams that drain toward the south or southeast. These streams thus had an erosive advantage during dissection of the landscape. This may explain the predominance of drainage captures by streams flowing toward the south or southeast. It may also explain why northwest-trending valleys (lineaments) are so prominent in the modern landscape.

Karst and Caves

Karst geomorphology is well developed in many parts of the Flint Hills, as a result of ground-water solution of thick limestone units. Common karst features in the Flint Hills include sinkholes, caves, and springs. Sinkholes are abundant in the Fort Riley Limestone in several portions of Butler County--Figure 14. Where present, sinkholes are quite numerous and conspicuous; a density of 10 or more sinkholes per quarter section is typical--Figure 19. Elsewhere sinkholes are generally lacking in the Fort Riley outcrop region. Zones of dense sinkhole development are associated with several structural and topographic conditions (Aber 1992).

These factors in combination are thought to enhance the possibility for vertical drainage of water into highly fractured, soluble bedrock with resulting sinkhole solution. Springs and caves are also known in association with sinkhole zones, especially in the region between El Dorado and Leon.

Large sinkhole in Ft. Riley Limestone near Smith Cave, Butler County. The people are standing within the cone-shaped sinkhole, looking down at the opening in the bottom. Photo date 10/95, © J.S. Aber.

Caves are fairly common within the Fort Riley Limestone of the Flint Hills, especially in central Butler County. Three caves and numerous sinkholes have been mapped in the karst terrain between El Dorado and Leon--Figure 19. Spring Cave has a surveyed length of > 8500 feet (> 2500 m); Smith Cave is > 7200 feet (> 2200 m) long, and Windmill Cave has > 670 feet (> 200 m) of mapped passages. The combined surveyed length of these caves is, thus, more than 3 miles (5 km).

The three enterable caves are connected by water-filled passages. Dye tracing has confirmed the hydrologic connections, which suggest an integrated ground-water flow system. Water input is via sinkholes and soil percolation, and the entire system drains westward to a perennial spring. Normal (low flow) discharge of the spring is approximately 200 gal/min (12½ l/s). As with many karst springs, Spring Cave is extremely flashy (flood prone) following heavy rainfall, when the spring may become a raging torrent several times the normal discharge. At such times, sediment transport and mechanical erosion are greatly increased within the cave system.

Spring Cave passages are strongly controlled by fractures--Figure 20. Passages are tall, narrow, solutionally enlarged joints that have sharp, angular bends at joint intersections. Passages follow the two main joint trends of the region at 50-70° and 300-320°. Two parallel passages--right and left--emerge at the spring entrance and are connected upstream to passages of Smith Cave. Smith Cave and Windmill Cave exhibit some joint control of their passage trends, but not so much as Spring Cave.

Smith Cave displays passages that are often controlled by bedding planes, especially in its eastern and central sections--Figure 20. The passages are sinuous and have elliptical shapes at the tops with relatively flat ceilings. The lower parts of passages are characterized by vadose canyons in which streams are actively downcutting. Most ground water in Smith Cave flows along the Main (northern) passage into the left passage of Spring Cave. However, the Y passage diverts about 25-30% of flow from Smith Cave into the right passage of Spring Cave. The Y passage once may have been a tributary of Smith Cave, but through underground stream piracy its direction of drainage is reversed now into Spring Cave (Bain 1992).

The entrance to Smith Cave is covered by the windmill, which pumps water directly from the cave passageway. A small opening below the windmill allows human access to the cave. Photo date 10/95, © J.S. Aber.
Students examine the entrance to Spring Cave, Butler County. This is one of the few Flint Hills' caves that can be entered standing up. Photo date 10/92, © J.S. Aber.

The Spring/Smith cave stream is inhabited by fairly abundant populations of albino isopods, amphipods, crayfish, and catfish. This diverse faunal assemblage suggests stability in water quality and also that these species probably have thrived for many generations in the cave environment (Bain 1992). In Smith Cave, catfish were identified as black bullheads (Ictalurus melas), and the predominant amphipod species appears to be Clanton's cave amphipod (Stygobromus clantoni).

Return to Flint Hills geology.
J.S. Aber © 2002.