The eastern edge of the Wellington-McPherson-Arkansas River Lowlands marks the divide between the Neosho-Cottonwood and Walnut basins to the east and the lower Arkansas drainage to the west (fig. 2). This edge has retreated westward, due to steeper gradients and more aggressive erosion in the headwaters of the eastern systems. At one time, the Arkansas alluvial plain must have sloped gently eastward. Isolated exotic pebbles on hilltops demonstrate the former extent of the alluvial plain. In like manner, the terrain east of the Flint Hills must have been considerably higher than today. For example, highest gravels of the ancestral Verdigris are preserved on the drainage divide in northeastern Greenwood County, at an elevation of 390 m (1,280 ft), 80 m (260 ft) above the modern Verdigris floodplain.
In previous studies, gravels on the Missouri-Arkansas divide in Anderson County were interpreted as an eastern extension of gravel trends along the Cottonwood and Neosho valleys (Frye, 1955; Aber, 1985). However, the regional slope of these gravels to the southwest argues strongly against this point of view. These gravels apparently represent a river flowing from the northeast. Origin of the headwaters for this stream is uncertain, as are the sources for exotics. The gravels are situated up to 70 m (230 ft) above regional floodplains.
At the time of deposition, chert gravels occupied the lowest topographic positions--stream channels--in the surrounding landscape. Preservation of exotic-bearing gravels in drainage-divide positions demonstrates a wholesale inversion of topography in eastern Kansas. Considerable erosion has taken place, such that former low points now occupy the highest positions in the local landscape. The minimum magnitude of vertical erosion can be estimated from the elevations of highest chert gravels in relation to present stream-valley floodplains in each drainage basin (table 1).
Table 1. Minimum amount of vertical erosion in eastern Kansas drainage basins.
| Drainage Basin | Erosion |
|---|---|
| Walnut | 80 m (260 ft) |
| Verdigris | 80 m (260 ft) |
| Marais des Cygnes | 70 m (230 ft) |
| Neosho | 50 m (160 ft) |
| Fall | 50 m (160 ft) |
| Cottonwood | 40 m (130 ft) |
These figures indicate at least 40 m to 80 m (130-260 ft) of vertical erosion has taken place across eastern Kansas during the Quaternary. The figures are minimum estimates only; they do not take into account deeper valley erosion and aggradation (below floodplain level). River entrenchment has been greatest in the western Flint Hills (Walnut basin), immediately east of the Flint Hills (Verdigris basin), and in the Marais des Cygnes basin, where 70-80 m (230-260 ft) of downcutting is demonstrated. This pattern of erosion suggests that the Flint Hills may have emerged gradually as a bedrock massif, while terrains to the east and west were eroded down.
Kansas Geological Survey
Web version March 18, 1998
http://www.kgs.ku.edu/Current/1997/aber/aber12.html
email:lbrosius@kgs.ku.edu