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Eldorado Oil and Gas Field

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Regional Structure

The dominating structural feature of the northern Midcontinent oil field is the Ozark uplift, a widespread regional elevation with maximum development in southwestern Missouri. This dominating uplift is the result of diastrophic activities which principally took place in two periods—the first in late Mississippian or earliest Pennsylvanian time, and the second in post-Paleozoic time. [Note: Diastrophic movements in the Ozark region and in eastern Kansas before the Mississippian are clearly indicated by the stratigraphic relations of the Silurian, Devonian and Mississippian strata about the Ozark highland, and the subsurface studies in east central Kansas conducted by the Kansas Geological Survey—Raymond C. Moore.] The first deformation involved the Cambrian, Ordovician and Mississippian strata and resulted in an extensive emergence, followed by a long period of erosion which affected principally the Mississippian rocks. This erosional interval is recognized in eastern Kansas through a widespread unconformity between the Mississippian and the overlying Pennsylvanian rocks-an unconformity which is marked, not by any notable angular divergence of the beds, except locally, as in the Eldorado field, but rather through the irregular eroded surface of the Mississippian strata, and the absence of lower Pennsylvanian strata such as are found in regions to the south.

The Pennsylvanian and Permian rock series deposited on the eroded Mississippian were not notably deformed until the second phase in the development of the Ozark uplift, which took place in post-Paleozoic times. This phase of the Ozark uplift consisted principally of a regional elevation, with its maximum development in the Ozark mountain region of Missouri. In all directions away from this central region the uplift extended with diminishing elevation, and throughout eastern Kansas and other bordering regions the rocks were only very gently tilted. In eastern Kansas the inclination of the strata due to the regional elevation is westward and amounts to between 15 and 25 feet to the mile. This inclination is indicated in a highly exaggerated form in plate VII.

Because eastern Kansas and the adjacent areas are not located in the heart of the Ozark uplift, the westward inclination of the rocks in these regions, although directly due to the uplift, is generally spoken of, not as a part of the Ozark uplift itself, but instead as the Prairie Plains monocline. Erosion has removed much of the Paleozoic sediments involved in the Ozark uplift, and in places in the center of the uplift the pre-Cambrian is exposed. In eastern Kansas on the flanks of this regional uplift the westward-tilted strata were bevel-ld, and are now exposed in broad parallel belts trending north-northeastward across the state.

The oldest rocks exposed in Kansas are of Mississippian age and are located in the southeast corner of the state. To the west-northwest follow the outcropping belts of the successively higher formation belonging to the Pennsylvanian and Permian divisions, which dip gently westward. This westward dipping attitude of the strata in eastern Kansas, the Prairie Plains monocline, is spoken of as the regional structure, and it is upon this that the local and minor irregularities, such as the anticlines, domes and synclines of the Eldorado field are developed.

The structural features of the Eldorado field will be described in considerable detail, but before doing so it seems pertinent to discuss briefly the relation which the structure of the surface rocks bears to the accumulation of oil and gas.

Relationship of Surface Structure to Oil and Gas Accumulation

The oil and gas in the Midcontinental field is almost invariably, if not always, associated with water, and where present in considerable amounts the oil and gas will tend to accumulate in the tops of the folds or arches into which the oil-bearing strata are flexed, in case they are uniform in thickness and porosity, and the water will lie beneath in the lower levels. This situation will obtain though the folds in the exposed beds are not parallel or identical with those in the sands. It is, however, the rule that in most oil and gas regions the underground structure of the oil- and gas-bearing rocks is reflected in the surface rocks by similar structural features; in other words, the surface rocks very generally lie in such positions that they closely parallel the underground rocks.

Obviously the structural geologic study of an oil and gas region is for the purpose of determining as accurately as possible the structure—that is, the position and magnitude of the folds—of the oil- and gas-bearing beds. It is also obvious that because of the impossibility of observations underground, the structure of the oil- and gas-bearing "sands" cannot be determined in detail previous to drilling. Therefore, while this information is unavailable the geologist must resort to the best accessible indications as to this underground structure, and from these draw his conclusions.

The best indications of the underground structure, i. e., the best symptoms upon which a diagnosis can be made—are to be found in the structural features of the outcropping rocks which so generally closely parallel the underground rocks. It is because the structure of the outcropping rocks furnish the best accessible surface indications of the subsurface structure that geologists give so much attention to the structure of the surface rocks. It is not a matter of choice, but of necessity.

The use of surface structure is not limited to the outlining of favorable acreage in advance of considerable drilling developments; it is of inestimable value in the further development of partly drilled fields, where to secure best results it is used in conjunction with the underground structural and "sand" information revealed locally by the preliminary drilling. Surface structure is of use in the active development of any field until the limits of the producing territory have been clearly defined by drilling. Thereafter, during the inside or intensive development of a field, it is of little value provided all the information that should be gathered as the drilling proceeds is accessible.

The study of surface structure is of value other than in the development of a field. The examination of its relationship, even in its details, to the underground structure and to the distribution of accumulated oil and gas as revealed in developed fields, leads to generalizations which should apply to other localities and should lead to the discovery of other oil and gas fields. If applied to prospective fields in the same general region, their economic and scientific development should be greatly advanced and expedited. It is expected that such will be the principal result of this detailed investigation of the Eldorado field, where the relationships of surface structure to underground structure and to occurrence of oil accumulation are rather clearly defined. For instance, the relationship between the surface structure and the location of the oil in the developed portions of the Eldorado field is advanced on pp. 143-147 of this report as a reason for believing that one of the Eldorado field domes, the Chelsea, which up to April, 1920, has failed to yield oil, has not been adequately tested.

The use to which a knowledge of the origin of oil-field structure may be put will be discussed farther on under the heading, "Origin of the Eldorado anticline and of other types of structure found in the northern part of the Midcontinent oil and gas field."

Structure of the Eldorado District

Structure of the Surface Rocks

Eldorado Anticline and Walnut Syncline. The structure of the exposed beds in the Eldorado field, which consists of minor and local modifications of the regional monoclinal structure, was ascertained by determining, with a plane table, telescopic alidade, and stadia, the distribution of the various strata and their altitude at more than 1,600 points scattered rather uniformly over the field. With this horizontal and vertical control the altitude of the top of the Fort, Riley limestone was determined throughout the field; where below the surface by subtracting the thickness of the intervening rocks, and where eroded by adding the known thickness of the rocks removed. These elevations were then used in drawing the structure contours given on plate I, which indicate the attitude or lay of the surface beds.

One major fold, the Eldorado anticline, extends in a somewhat sinuous line and in a southwesterly direction across the entire area embraced in this report, from the southeast corner of T. 24 S., R. 5 E., to beyond the southwest corner of T. 26 S., R. 5 E. According to the map published by McDowell (1917, p. 294), the southward extension of the Eldorado anticline continues into T. 27 S., R. 4 E., to near the town of Augusta; and it is further possible that to the north it extends beyond the northern limits of the Eldorado district as mapped on plate I, in which case the Lincoln syncline is but a minor cross fold separating the Chelsea dome from the dome next beyond. The Eldorado anticline is marked by alternating minor domes and minor cross synclines, and at its southwest end is forked, with the Shumway dome and Whitewater nose forming one branch and the Boyer dome and Koogler nose forming the other. On several of the domes which modify its crest, erosion has exposed the lowermost strata of the district, the Florence flint, which crops out in irregular-shaped areas surrounded by the Fort Riley limestone. The Eldorado anticline is limited on the east by the pronounced Walnut syncline, which is located in the main valley and west branch of Walnut river, after which it is named. This syncline is a rather straight trough extending from beyond the south border of the Eldorado district, to the east-central part of T. 25 S., R. 5 E. Here it terminates in a slightly accentuated depression from which minor synclines radiate to the northwest and east. The maximum vertical difference between the crest of the Eldorado anticline and the trough of the Walnut syncline is about 180 feet. Because of its location in the valleys of Walnut river and its west branch, the rocks exposed in the Walnut syncline belong, for the most part, to the lower portion of the exposed stratigraphic section, and in but one locality, that of the accentuated depression in the east-central part of T. 25 S., R. 5 E., are rocks as high as the Towanda limestone and the Winfield limestone present. The minor domes and minor synclines which modify the Eldorado anticline are several in number, and to facilitate discussion they will be designated by names. The names as given below are taken either from local features or from the more prominent leases located on them. They may be classified as follows:

Minor domes: Minor synclines:
Shumway dome with Whitewater nose extension. Hammond syncline.
Boyer dome with Koogler nose extension. Bishop syncline.
Oil Hill dome. Fowler syncline.
Chesney dome. Bancroft syncline.
Wilson dome. Hegberg syncline.
Robinson dome. Dunkle syncline.
Chelsea dome. Ramsey syncline.
  Theta syncline.
  Lincoln syncline.

Shumway Dome. This dome is located principally in the northeast corner of T. 26 S., R. 4 E. (Towanda township), where the Fort Riley limestone outcrops over most of its crest. It is slightly elongated in a northeasterly direction, extending from the SW, sec. 31, T. 25 S., R. 5 E., on the northeast, to sec. 14, T. 26 S., R. 4 E., on the southwest, a distance of about three miles. Its width ranges from 1 1/2 to about 2 miles. Because of the lack of well-defined rock exposures, the exact location of its apex is not clearly indicated. It seems, however, to lie near the northeast corner of section 11, which also is the northeast corner of the prolific Shumway lease, after which the dome is named. The elevation of the apex, referred to the top of the Fort Riley limestone, is over 1,420 feet above tide and its closure amounts to about 30 feet. A plunging anticlinal nose, apparently related to the Shumway dome, from which it offshoots to the south, is located in secs. 22, 27 and 33, T. 26 S., R. 4 E. This plunging fold is called the Whitewater nose, after Whitewater river, which is located on its west flank.

The most prolific production of the entire field, almost altogether from the Stapleton pay zone, has been developed on the Shumway dome in section 11, a short distance southwest of the crest. Production from this pay is continuous across the shallow Fowler syncline to the east, and to the south and west the limit of production is roughly marked by the 1,340-foot contour (plate I), which is about 80 feet below the crest. To the northwest the limit of the producing area on this dome rises gradually, and in sec. 35, T. 25 S., R. 4 E., it lies within the 1,360-foot structure contour. It is to be noted that the 1,600-foot or Boyer "sand," which has been developed over a part of this dome, is productive only on the crest of the dome, and that this is the only locality where the 1,475-foot gas "sand" has been exploited commercially. The 900-foot gas "sand" is productive at the north end of the Shumway dome and a few scattered 1,125-foot and 1,200-foot "sand" gas wells have also been obtained.

In the above discussion the distribution of the oil and gas has been described with reference to the contours on plate I, which show the structure of the surface rocks. In the chapter describing the oil and gas sands, structure contours on the 660-foot oil sand and the Stapleton oil zone, given on plate XIV, will be referred to in connection with those and other sands. It will be noted in these later discussions that the production from the Stapleton pay zone will receive the principal attention, its relation being considered of greater significance to the development of other fields in the surrounding region.

Boyer Dome. The Boyer dome is a quadrilateral-shaped area centering along the boundary of sections 8 and 17 of T. 26 S., R. 5 E., near the Boyer lease, and covering most of sections 7, 8 and 17, and extending into sections 5, 6, 9, 16 and 18 of the same township. Its crest at the surface is developed in a rather broad expanse of the Florence flint. Its apex, as indicated on plate 1, which refers to the top of the Fort Riley limestone, has an altitude of about 1,440 feet. This is about 20 feet higher than the Shumway dome. Its closure also is correspondingly greater, amounting to about 50 feet.

At the time the field work was completed developments had not clearly defined the limits of the oil and gas production on this dome. The Stapleton pay zone production is continuous with the Shumway and Oil Hill domes to the west and north, and extends far down on the nose in the direction of the southeast corner of the township. On the south and west sides of this nose, which will be referred to as the Koogler nose, after the principal lease on which it is located, production is found as low as the 1,310-foot contour. To the east of the dome proper, along the north side of section 9, and down on the flanks of the anticline, Stapleton oil production has been obtained as low as the 1,360-foot contour, but farther south in section 16 it appears to be considerably higher. It is to be noted that the 1,600-foot or Boyer "sand" production and the gas production from the 900-foot, 1,125-foot, 1,200-foot and 1,275-foot "sands" are limited to the crest of the dome.

Oil Hill Dome. The Oil Hill dome is slightly elongated in a northwesterly direction and centers in the north-central part of sec. 33, T. 25 S., R. 5 E., near Oil Hill. Its crest at the surface is developed in the Florence flint, which crops out over most of section 33 and small parts of neighboring sections. The Oil Hill dome is separated by the Bancroft syncline from both the Boyer and Shumway domes and its apex lies at the same elevation as that of the Boyer dome, about 1,440 feet above sea level. Its closure amounts to over 25 feet.

The 660-foot Stokes and Stapleton oil pays and several of the gas "sands" are productive on this dome. The extent of the producing area in the Stapleton oil zone is peculiar, in that at the southeast it extends down to the 1,330-foot contour, while in the northern part of sec. 34, T. 25 S., R. 5 E., it is about 30 feet higher, or near the 1,360-foot contour. To the west and northwest it is still higher, although its limits in this direction were not definitely outlined at the time this report was written. The lower extension of the oil on the east side is a condition prevailing also on the Wilson and Robinson domes farther north, and probably also on the Chesney dome. The oil production in the 660-foot sand is rather symmetrically distributed structurally, being limited in general by the 1,380-foot contour. The gas production is.practically confined to the area outlined by the 1,400-foot contour. The development of the Stokes "sand" as given on plate I is confined to a few wells near the east quarter corner of sec. 33, T. 25 S., R. 5 E., and it is to be noted that this is the only locality in the field where this sand was exploited.

Chesney Dome. This dome is nearly quaquaversal in shape. It centers in the SE, sec. 21, T. 25 S., R. 5 E., on the Chesney lease and extends into the adjacent quarter sections. The shallow Hegberg syncline separates it from the Oil Hill dome. The crest of the dome is surfaced by the Florence flint. The closure of the Chesney dome probably does not exceed 20 feet, and the elevation of its apex, referred to the Fort Riley limestone, lies about 1,430 feet above tide, which is 10 feet lower than that of the Oil Hill dome.

The 660-foot and Stapleton oil pays and the 1,275-foot gas pays are productive on this dome. The Stapleton zone has not been thoroughly exploited in this dome; but even so, its extent down on the east flank is noteworthy, where it is found at about 1,330-foot contour, which is 100 feet below the apex. On the other hand, the shallow or 660-foot sand has been almost completely developed and in it the oil shows a unique distribution. Near the south quarter corner of section 22 it is limited by the 1,360-foot contour; but to the north, in the NW of this same section, it rises to the 1,380-foot contour, and still farther north it drops rapidly to the 1,360- and 1,350-foot contours in sections 15 and 16. On the west side it rises from north to south from the 1,360-foot contour in section 16 to about 1,385 feet in the southwest corner of section 21. The gas which is produced from the 1,275-foot "sand" is confined to the area within the 1,390-foot contour.

Wilson Dome. This dome is elongated with a west-northwesterly trend, and although centering in sec. 8, T. 25 S., R. 5 E., includes also parts of sections 4, 5, 6, 7, 9, 16 and 17 of the same township. Its crest is considerably lower than those of the domes to the south, and as a result the surface rocks on this feature are principally the Doyle shale, with the top of the Fort Riley exposed only in small areas. Its apex referred to the top of the Fort Riley limestone has an altitude of more than 1,385 feet and its closure probably exceeds 40 feet. It produces oil from both the 660 foot and Stapleton pays and gas from the 1,200- and 1,275-foot "sands."

The distribution of the oil in the Stapleton zone is to be noted. To the southeast it is as low as the 1,320-foot contour. Near the north quarter corner of section 9 it is at about 1,355 feet, and in the southeast corner of section 6 it is about 1,360 feet. This lower position of the oil on the southeast side is similar to that pointed out for the Oil Hill dome and will also be pointed out for the Robinson dome to the east. This same peculiar distribution does not apply for the oil in the shallow sand, whose limits have not been fully exploited, and therefore cannot be definitely outlined. The gas which has been developed lies well within the area outlined by the 1,370-foot contour, or within 15 feet vertically of the apex.

Robinson Dome. This low northeasterly trending structural feature is surfaced by the Fort Riley limestone and Doyle shale, and is located almost entirely in sec. 3, T. 25 S., R. 5 E. The altitude of its apex is about 1,375 feet, the lowest of the Eldorado domes, and its closure is probably less than 25 feet. The Stapleton oil zone is the only pay on this dome. The outstanding feature of the distribution of the productive area is that its western limit is less than 10 feet below and less than one-eighth mile distant from the apex of the dome, whereas to the east it persists for at least a mile into and beyond the Ramsey syncline and lies a short distance up on the southwestern extension of the Chelsea dome. The structure of the outcropping rocks does not suggest this great eastward extent, of production which was developed since the completion of the field studies. It is possible that the structure of the Stapleton sand controls this distribution, but, unfortunately, there is no detailed underground information available for this report by which this can be determined.

Chelsea Dome. This elongated structure may be termed a minor anticline, but for reasons of uniformity it is classed as a dome. Its surface development is principally in the lower member of the Doyle shale, although tongues of the Fort Riley limestone indicate the configuration of its southeast and northwest ends. The Towanda limestone crops out on its south flank, and this limestone in conjunction with the Winfield limestone determines its north flank. The long axis of the dome extends from the center of sec. 35, T. 24 S., R. 5 E., to the center of sec. 8, T. 25 S., R. 6 E., a distance of about three and one-half miles. It includes all or parts of secs. 35 and 36, T. 24 S., R. 5 E.; sec. 31, T. 24 S., R. 6 E.; secs. 1 and 2, T. 25 S., R. 5 E.; and secs. 5, 6, 7 and 8, T. 25 S., R. 6 E. The altitude of its crest, referred to the top of the Fort Riley limestone, exceeds 1,390 feet and its closure amounts to more than 40 feet.

No oil has been found in the Chelsea dome except as noted in the discussion of the Robinson dome. Three dry holes have been drilled on it in widely scattered places, none of which, according to the opinion of the writer, were located in the most favorable place. One of these dry holes is in the NW NW, sec. 1, T. 25 S., R. 5 E., another in the SE SE, sec. 36, T. 24 S., R. 5 E., and the third in the southeast corner of the SE, sec. 1, T. 25 S., R. 5 E. The inadequacy of these tests will receive special consideration (pp. 143-148) after the relation of the oil with reference to the structure of the Stapleton pay zone has been discussed.

Hammond Syncline. Outside of the Walnut syncline, which is the major depression marking the eastern limit of the Eldorado anticline, the most prominent feature of this type is the Hammond syncline, a deep local depression, much deeper than the Walnut syncline, named after the Hammond lease which it crosses. This depression centers in sec. 34, T. 26 S., R. 4 E., and its southeastward extension, according to McDowell (1917, p. 294), separates the south extension of the Eldorado anticline from the North Augusta anticline of the Augusta field. The Herington (?) member of the Marion formation is the key horizon indicating the depth of this depression, and the Winfield limestone outlines its flanks. The top of the Fort Riley limestone in the trough is certainly below an altitude of 1,140 feet, and may be as low as 1,120 feet or even less. The sharpness of this depression has resulted in a marked reversal of dip on the west, producing there a plunging anticlinal flexure, the Whitewater nose, which is a southwesterly offshoot from the Shumway dome.

Bishop Syncline. Located in a direct line with the axis of the Hammond syncline, and also possibly genetically related to it, is another depression. extending across the Bishop lease in sec. 23, and into SW, sec. 13, T. 26 S., R. 4 E. This depression permits the Winfield to crop out in the vicinity of the southwest corner of section 13, and the elevation of the Fort Riley in the lowest portion of the trough is below 1,265 feet. This depression is sufficiently developed to exclude the presence of oil in its trough.

Fowler Syncline. Still farther north, and located in line with the Hammond and Bishop synclines, to which it appears to be genetically related, is the Fowler syncline, extending from the Fowler lease in sec. 1, T. 26 S., R. 4 E., into section 13 of the same township, and into sec. 6, T. 26 S., R. 5 E. The Fowler syncline separates the Shumway and Boyer domes and in its trough is a small outcrop of the Doyle shale, which is exposed along the railroad in sec. 1, T. 26 S., R. 4 E. Production is continuous across this shallow depression from the Shumway to the Boyer domes.

Bancroft Syncline. This shallow downfold, trending northwesterly from sec. 4, T. 26 S., R. 5 E., to sec. 30, T. 25 S., R. 5 E., crosses the Bancroft lease and separates the Shumway and Boyer domes from the Oil Hill dome. Due to its slight development it appears to have had little or no effect in controlling the accumulation of the oil, production being continuous from the Oil Hill dome on the north to the Boyer and Shumway domes to the south and southwest.

Hegberg Syncline. This depression, located on the Hegberg lease, separates the Oil Hill and Chesney domes. It is very shallow and apparently has had no appreciable effect in controlling the oil accumulation.

Theta Syncline. A pronounced depression, indicated principally by the Towanda limestone member, crosses leases of the Theta Oil Company in secs. 16 and 17, T. 25 S., R. 5 E., and separates the Chesney dome from the Wilson dome. It is a westward extension of the deep basin of the Walnut syncline in the east-central part of this township and is sufficiently developed to separate in both the shallow and Stapleton zones the production of the Chesney and Wilson domes.

Dunkle Syncline. Starting on the Dunkle lease in sec. 4, T. 25 S., R. 5 E., this depression trends southeastward across section 9 to the Walnut syncline and separates the Wilson and Robinson domes. Like the Theta syncline, the Dunkle syncline also separates the production of the domes flanking it.

Ramsey Syncline. Located north of the Ramsey station on the Atchison, Topeka & Santa Fe railway is a shallow depression separating the Robinson and Chelsea domes. Lying in the valley of the west branch of Walnut river, where rock outcrops are absent, its configuration along the line between sections 2 and 3 is not discernible. Although a syncline, much production has come from it, while wells located higher structurally, according to the surface rocks, on the as yet unproductive Chelsea dome to the east failed to find production. This development of oil in the Ramsey saddle could probably be explained were all the information available, but as this development took place after the completion of the field work, no explanation is here presented.

Lincoln Syncline. Marking the north and northeastern boundaries of the Chelsea dome is a syncline, well indicated by good exposures of the Winfield limestone. It is located principally in Lincoln (political) township, which extends across sec. 25, T. 24 S., R. 5 E., and secs. 30, 31, 32 and 29, T. 24 S., R. 6 E. The region to the north was not examined, but it is possible that the Eldorado anticline continues in this direction.

Structure of Underground Rocks

The structural relation of the underground rocks to those outcropping at the surface, as previously pointed out, is of foremost importance in understanding the conditions which control the accumulation of oil and gas into oil pools. To recapitulate briefly, it is patent that to cause the accumulation of oil in anticlines and domes, these structures must be found in the oil- and gas-bearing rocks, and it is only where anticlines and domes developed in the oil- and gas-bearing rocks affect also the surface rocks that structural investigations in advance of developments are of use in directing exploratory work. That these conditions prevail over so much of the oil and gas regions explains the apparent ease with which geology has in the past assisted the oil industry in the locating and developing of oil and gas pools. If the structure in the oil sands always conformed to that of the surface rocks, and if the surface rocks were also of such character as to lend themselves to structural studies, geologists would have little trouble in locating all oil and gas pools in which folding was the factor causing the oil accumulation. In many places, however, these simple conditions do not prevail, and herein lie the difficulties which must be surmounted by much painstaking work and the accumulation of many details of fact.

The study of the underground structural conditions in the Eldorado field was undertaken to show what relation the structure of the surface rocks bears to the structure of the buried rocks, principally the oil- and gas-bearing beds. The condition prevailing at Eldorado should shed light on what may be expected in other localities of the same general region. As a result of the studies it is evident that the structure of the underground rocks is similar to that in the surface rocks, but that the relation between the two structures is not ideally simple. Further, the conditions prevailing in the Eldorado field are found to indicate, as will be demonstrated, that the number of localities where the development of new oil pools may be expected is greater than has in the past generally been considered probable.

Sufficient drilling information was obtained to determine the structure of the oil- and gas-bearing beds in a portion only of the producing area. The results obtained are graphically shown on the structure contour maps given on plate XIV and figures 3 and 4. The discussion of their structure, however, instead of being given at this point, is presented under the next general heading as a means of simplifying the general treatment of the sands.

The structure of the underground rocks is described as follows:

660-foot oil sand pp. 77 to 78
1,275-foot gas "sand" p. 81
900-foot gas "sand" pp. 81 to 83
Stapleton oil zone pp. 99 to 132
Smock-Sluss pool pp. 138 to 139
Origin of structural features pp. 148 to 166

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Kansas Geological Survey, Geology
Placed on web July 28, 2017; originally published 1921.
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