Cenozoic EraTertiary Period
Prior to the deposition of Tertiary sediments there was a period of folding during which the major features of the bedrock depression in Scott and Finney counties may have been formed. A broad asymmetrical trough, with its axis extending from Garden City to Scott City and northward, was developed. At the beginning of Tertiary time an extensive land surface existed in western Kansas; this surface was subjected to erosion largely effected by through-flowing streams. During this interval great thicknesses of Upper Cretaceous sediments were removed, so that in Scott County all Cretaceous strata above the Niobrara are missing and variable thicknesses of the upper part of the Niobrara have been removed. The present shape of the trough in cross section is shown by sections A-A', B-B', and C-C' in Figure 5, and the approximate configuration of the pre-Tertiary surface possibly modified by post-Tertiary erosion is shown in Figure 2.
Figure 5--A, east-west geologic profile along Kansas Highway 96 through Scott City; B, east-west geologic profile along section road 0.5 mile south of Shallow Water; C, north-south geologic profile across the middle of Scott County. A larger version of this figure is available.
Some crustal deformation may have taken place after the deposition of the Ogallala, probably along pre-established lines of weakness.
Pleistocene Epoch--A period of erosion preceded the beginning of the Pleistocene during which the pre-Pleistocene surface was subjected to subaerial erosion. In early Pleistocene time, streams were rejuvenated as a result of uplift to the west and because of climatic changes, so that erosion followed by sedimentation was resumed. The trough which had been deepened by downwarping or erosion or both was filled with sands and gravels which in turn were mantled by windblown loess. The sand and gravel deposits were laid down by heavily laden streams that probably shifted laterally at frequent intervals. Stream deposition was followed by eolian activity, indicated by the fact that the fluvial deposits grade upward into loess. This seems to indicate that during the latter part of the Pleistocene there was a climatic change with considerable wind movement. Loess deposition has continued intermittently until Recent time.
Recent Epoch--At the beginning of the Recent Epoch, streams began the downcutting that has produced the present topography. It was during this period also that the courses of the several streams were established. The presence of a Cretaceous ridge in eastern Scott County has affected the behavior of streams traversing the county. Beaver Creek or Ladder Creek, a tributary of Smoky Hill River, enters the county from the west and flows eastward to a point about midway across the county from east to west, where it turns abruptly northward, continuing in this direction to its junction with the Smoky Hill. This anomalous change in direction is probably directly related to the structure of the underlying bedrock. Whitewoman Creek is another notable example of a stream whose course has been affected by the position of the underlying bedrock. This stream differs from most other streams in that it has no connection with any other stream, and in times of flood it empties its water into the broad shallow depression at its terminus, known as the Scott Basin.
Terrace deposits of sand and gravel are found in a belt immediately bordering the floodplain of Beaver Creek. Much of the material in the terrace gravels was derived from source areas to the west and was deposited by Beaver Creek at some time during the Pleistocene when it was flowing at a higher level than at present. The terraces are of cut-and-fill origin and probably are of late glacial or post-glacial age.
Dune sand mantles the surface in the southeastern part of the county (Pl. 1). Although its age is not definitely known, it is believed that accumulation of some of the sand started in late Pleistocene time and continued until Recent time. The sand originated possibly from eroded slopes cut in the Ogallala formation and from the Pleistocene sands and gravels. It is possible that the sand was derived either wholly or in part from the strand flats of a lake that formerly occupied the depression now known as Dry Lake. The dune-building winds of the past were in a northerly direction, whereas those of the present time are predominantly southerly. Smith (1940, p. 168) suggested that the presence of a continental ice sheet during one or more of the Pleistocene glacial stages would have provided ready cause for altered wind direction.
Kansas Geological Survey, Scott County Geohydrology|
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Web version March 2003. Original publication date July 1947.