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Riley and Geary County Geology

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Stratigraphy, continued

Cretaceous System

Dakota Group

Coarse ferruginous quartz sandstone, identified as belonging to the Dakota Group, is present on the uplands in the extreme northwestern portion of Riley County. This rock lies disconformably upon Permian strata and in one exposure can be seen to rest upon a few inches of limestone approximately 45 feet above the Nolans formation. Exposures of Dakota sandstone in situ in the two counties are believed to be confined to secs. 1 and 2, T. 6 S., R. 4 E., in Riley County. The contact between Permian and Cretaceous sediments is shown on the geologic map (pl. 1). About 2 miles west of Junction City in Geary County the extremely sandy soil on the uplands above the Fort Riley limestone seems to be residue from weathered Dakota sandstone and the same kind of soil is present on uplands above the Towanda limestone about 5 miles northwest of Junction City. It is possible that Cretaceous beds are concealed by soil in those places. It should be noted that Hay (1896, p. 23) reported finding Dakota sandstone 2 miles west of Junction City and showed two areas marked "Cretaceous outcrop" a short distance south of Junction City on the map accompanying his report on the Fort Riley Military Reservation. It is possible that bed rock has been transformed into sandy soil during the years since Hays studied there. The known facts, however, point to the conclusion that erosion had lowered the Paleozoic beds almost to the horizon of Fort Riley limestone or even lower before Cretaceous deposition, and that the Dakota sediments overlap the Permian from the west.

Tertiary System

During my field studies in the two counties I did not identify Tertiary sediments, but it must be mentioned that Hay reported rocks younger than Cretaceous occurring as bed rock within this area. In his paper of 1896 he described a "mortar-bed-like conglomerate" resting upon shale, which lies probably in the upper part of the Garrison formation. He reported it to be 30 feet above a river bed and east of the 97th meridian and within the area of his report, the limits of which are somewhat indefinite, but enclose only the vicinity of Fort Riley. He concluded that the deposit is "pre-loess if not actually Miocene." I have seen well-cemented gravel, which is mostly flint, in the higher parts of the valley fillings near the major streams in Riley and Geary counties. Such gravels and conglomerates now form the stream terraces, which are higher than the more recent alluvial deposits, and I regard them as being Pleistocene in age.

About 40 feet above drainage in the valley of Deep Creek in southeastern Riley County south of Zeandale is a terrace consisting of several feet of water-worn flint gravel. This material is overlain by a deposit of northern erratics of undoubted Pleistocene age and it is probable that the flint gravel bed is an earlier Pleistocene deposit.

Quaternary System--Pleistocene Series

The area described in this report lies along the border of the glaciated plains and evidence of ice invasion is not lacking. The area in which glacial drift is present is not extensive and no attempt was made to show drift on the map showing areal geology. Drift is present, in two areas in conspicuous quantities, both as fine material and as great boulders of quartzite as large as 12 feet in diameter. One is an area in Riley County south and east of Deep Creek in secs. 27 and 34, T. 10 S., R. 6 E. There the drift overlies Tarkio limestone and a flint gravel deposit. Boulders of deeply weathered quartzite as large as 12 feet in diameter occur there. The other is an area in the northeastern part of Riley County, and extends southward skirting the eastern edge of the uplands above Big Blue River as far as Manhattan. No glacial till was identified and no striae on bed rock were observed, so direct evidence of ice invasion is lacking, but it is very probably that the large boulders were actually deposited by ice.

The glacial drift was probably deposited in the Kansas stage of Pleistocene time, although the deep weathering of quartzite boulders might suggest earlier deposition, i.e., the Nebraska stage.


Loess, wind-blown clay and fine sand, in large or small quantities is associated with almost all of the larger streams in the area and especially in the part north of Kansas River. The loess is wind blown material, which was probably first deposited on the flood plains of streams swollen by the melting of glacial ice, but which were later moved and redeposited on higher ground by the wind. The loess is not restricted to that part of the two counties that is north of Kansas River, but the greater amount of it is to be found there. The largest areas covered by this material are situated near Manhattan, Ogden, and Fort Riley. There the loess generally modifies the prevailing type of topography and partly obscures the exposures of harder rock. The thickness of the loess is probably nowhere more than 50 feet and generally is much less. This loess contains an appreciable amount of very fine sand, estimated at 15 percent or less, and most of the rest is silt and clay. An area lying north of Ogden on each side of Sevenmile Creek and extending westward beyond Threemille Creek is the largest expanse of loess-covered territory within the two counties. The present altitude of the loess ranges between approximately 1050 and 1200 feet above sea level. It constitutes one of the best kinds of soil in this part of Kansas.

In Riley County the loess has been mapped (Carter and Smith, 1908) with the other soils and it is called "Marshall silt loam." In the report that accompanies the soil map it is said to be an alluvial deposit and is explained as being a lacustrine-lake deposit. The absence of bedding and the sizes of the particles, however, lead me to believe that this material was deposited by wind. Similar deposits are still accumulating although the source is mostly cultivated fields rather than river flood-plain deposits. The loess is probably partly Pleistocene and partly Recent in age. As mentioned in another part of this report, there are remnants of former valley fillings that may be in part lake deposits. These are bedded and are now exposed in terraces above the present rive flood plains; hence they differ from those that are described as being loess.

A deposit of water-worn flint gravel several feet thick in Deep creek valley in southeastern Riley County, has been shown on the accompanying geologic map along with other alluvial deposits. As explained, these flint gravels are probably early Pleistocene in age.

Recent Deposits

On the map showing areal geology (pl. 1) the flood-plain deposits of the major streams are shown as alluvium. As stated elsewhere in this report, almost all of the streams have developed flood plains, but the nature of the deposits along the smaller streams is very different from the sandy alluvial material in the flood plains of Kansas, Big Blue, Republican, and Smoky Hill rivers. In the valleys of the smaller streams the flood-plain material has been accumulated primarily by the downwash of weathered shale and loess from the valley walls, although in part It comprises material deposited when more active sedimentation was in progress during and soon after the glaciation of the area not far to the northward. In the valleys of the major streams, the alluvium near the surface is silt, very fine sand, and sand, but at greater depth coarser material predominates. The average thickness of alluvial deposits along the larger streams is perhaps 50 feet. The thickness is generally much less in the valleys of the smaller streams. This material occupies narrow belts along the streams and these belts range in width from a few scores of feet to a few miles.

The residual soil, which is described hereinafter under the heading Economic Geology, is a part of the recent deposits of the two counties, and as previously explained, the loess is partly of Recent origin.

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Kansas Geological Survey, Geology of Riley and Geary Counties
Web version Nov. 2000. Original publication date Dec. 1941.
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