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AAPG Bulletin


Volume: 16 (1932)

Issue: 2. (February)

First Page: 119

Last Page: 134

Title: Lower Permian Correlations in Cleveland, McClain, and Garvin Counties, Oklahoma

Author(s): Robert H. Dott (2)


New correlations of the subdivisions of the Enid group of McClain and Cleveland counties, with the stratigraphic units described in Garvin County, are proposed. The base of the Permian is placed at the base of the Asher formation of Morgan, and at the base of Unit 1 of Dott, rather than at the Hart limestone, as proposed by Morgan. The Stillwater formation is thought to be equivalent to Unit 1 in Garvin County, and the Wellington formation to Unit 2. The Garber formation is thought to include Units 3-6 inclusive, and the Hennessey formation to include Units 7 and 8. These correlations are based on lithologic similarity, the sequence of beds, similar thicknesses, and on the presence and position of such markers as zones of barite rosettes and pseudo-conglomerates.



This article was originally prepared for publication by the Oklahoma Geological Survey. The figures were prepared in the Survey office. This opportunity is taken to thank C. L. Cooper, chief geologist, for his valuable help and criticism, and to thank Charles N. Gould, director, for his permission to publish the material in this Bulletin.


The extension of drilling into the Permian area of south-central Oklahoma during the past few years has led to a more intensive study of the Red-bed formations, and many of the perplexing problems of stratigraphy are rapidly progressing toward solution.

End_Page 119------------------------------

In 1926, Aurin, Officer, and Gould (FOOTNOTE 1) re-defined the old term Enid, raising it from the rank of formation to that of group. They subdivided it into six members, and correlated them with the Kansas formations as shown in Table I.

It is understood that more recent work has necessitated several changes in this correlation, and that revisions will be made. These, when published, will be the results of several years of study by field geologists working in western Oklahoma.

The map of Aurin, Officer, and Gould showed the areal distribution of these formations from the Arbuckle Mountains northward. The correlations in the southern area were less definite than in the northern, and the contact lines as mapped were tentative. The Stillwater of the northern area was thought to include the Asher, Konawa, and Stratford formations of Morgan.(FOOTNOTE 2)

In 1927, Clark and Cooper (FOOTNOTE 3) described the formations in Kay, Grant, Garfield, and Noble counties. They followed Aurin, Officer, and Gould,


FOOTNOTE 1. F. L. Aurin, H. G. Officer, and Charles N. Gould, "The Subdivision of the Enid Formation," Bull. Amer. Assoc. Petrol. Geol., Vol. 10, No. 8 (August, 1926), p. 791.

FOOTNOTE 2. George D. Morgan, "The Geology of the Stonewall Quadrangle," Bur. of Geol. Bull. 2 (1924).

FOOTNOTE 3. G. C. Clark and C. L. Cooper, "Oil and Gas Geology of Kay, Grant, Garfield, and Noble Counties," Oklahoma Geol. Survey Bull. 40-H (1927).

End_Page 120------------------------------

except that they put the base of the Stillwater at the base of the Neva limestone, instead of at the base of the Cottonwood.

At nearly the same time, the writer (FOOTNOTE 1) described the Red-beds of Garvin County, giving an eight-fold subdivision of the beds from the top of the Pontotoc to the base of the Duncan sandstone (Fig. 1). An attempt was made to correlate these eight units with the subdivisions of Aurin, Officer, and Gould, but this correlation, unfortunately, was in error, and should be disregarded.

Later, Anderson (FOOTNOTE 2) described the geology of Cleveland and McClain counties, following the classification and mapping of Aurin, Officer, and Gould, with more detailed tracing of formation boundaries. His classification is considered by the writer as generally correct, except for the position of the base of the Stillwater formation. In that article, Anderson proposed a correlation of the beds in Garvin County with his subdivisions in McClain and Cleveland counties. It is this correlation which the writer wishes to discuss.

In Garvin County, 1,135 feet of Red-beds were measured with rod and plane table, from the top of the Pontotoc formation to the base of the Duncan sandstone. In McClain County, Anderson gives 1,340 feet (1,090 feet without the Konawa) for his pre-Duncan Enid, and in Cleveland County, without the Stillwater, approximately 1,400 feet. In the northern area, the total thickness of these rocks is 2,470 feet.


Aurin, Officer, and Gould, and Anderson have accepted Morgan's (FOOTNOTE 3) determination of the base of the Permian as the Hart limestone, which marks the base of the Stratford formation of the Pontotoc terrane, and have included Morgan's Konawa and Stratford formations in the Stillwater. This the writer is unwilling to do.

Morgan placed the top of the Pennsylvanian within his Pontotoc terrane, which he subdivided as follows.


Pontotoc terrane
Konawa formation
Stratford formation
Vanoss formation

FOOTNOTE 1. Robert H. Dott, "Geology of Garvin County," Oklahoma Geol. Survey Bull. 40-K (1927).

FOOTNOTE 2. G. E. Anderson, "Geology of Cleveland and McClain Counties," Oklahoma Geol. Survey Bull. 40-N (1927).

FOOTNOTE 3. George D. Morgan, op. cit.

End_Page 121------------------------------

Fig. 1. Composite geologic section, Garvin County, by Robert H. Dott. From "Geology of Garvin County," Oklahoma Geol. Survey Bull. 40-K (1927), p. 13, Fig. 2. Also "Oil and Gas in Oklahoma," Oklahoma Geol. Survey Bull. 40, Vol. II (1930), p. 125, Fig. 19.

End_Page 122------------------------------

He was in considerable doubt as to the relationship of the Konawa and Stratford formations, but concluded that the Konawa overlapped the Stratford. The writer believes the two to be equivalent, as previously indicated.(FOOTNOTE 1) Morgan found a species of the plant genus Walchia near the top of the Vanoss formation, and it is principally on this basis that he refers the Stratford and Konawa to the Permian. He states that David White said: " ...although a Walchia, it (Morgan's specimen) is not clearly the species Walchia pinniformis," though comparable with that species.

Walchia pinniformis has been reported by Sellards (FOOTNOTE 2) and White (FOOTNOTE 3) from the Wreford and Wellington in Kansas.

Morgan further quoted David White, as follows.

There are a number of cases in which it is clear that Walchia is present in the uppermost horizons of the Pennsylvanian. Yet it is in general so characteristic of the Permian that each such case deserves special inquiry.

In Kansas, the upper Pennsylvanian is known as the Wabaunsee formation, and the overlying lower Permian as Council Grove, Chase, and Marion formations. In Oklahoma the latter three formations are grouped into the Stillwater.

In Kansas, these late Pennsylvanian and early Permian formations are typically marine fossiliferous shales and limestones, and this facies continues for a considerable distance into Oklahoma. After more than 20 years of work the Permian-Pennsylvanian boundary is not yet universally accepted even in this fossiliferous series.

The red shales of the Pontotoc suggest continental origin, and the color change in central Oklahoma which cuts across the Permian into the Pennsylvanian, suggests that land conditions appeared earlier in the southern area than in the northern. Could not this earlier appearance of land conditions have permitted a slightly earlier appearance of plants with a Permian facies in the Pontotoc, without indicating positively Permian age? Attention may be called to the well-known tendency of floras to precede faunas in many geologic periods.

As Morgan's Walchia is not identical with the typical Permian W. pinniformis, is there not some better evidence for drawing the Pennsylvanian-Permian contact at some other horizon?

FOOTNOTE 1. Robert H. Dott, op. cit., p. 10.

FOOTNOTE 2. E. H. Sellards, "Fossil Plants of the Upper Paleozoic of Kansas," Kansas Univ. Geol. Survey, Vol. 9 (1908).

FOOTNOTE 3. David White, "Stratigraphy and Paleontology of the Upper Carboniferous Rocks of the Kansas Section," U. S. Geol. Survey Bull. 211 (1903).

End_Page 123------------------------------


Morgan stresses the presence of arkosic material in the Pontotoc as a characteristic of the group. It is not found lower, and none has been found above the base of Morgan's Asher formation, or of Unit 1 in Garvin County.(FOOTNOTE 1) The sandstones above that point are different in character, and probably in origin. The shales are considerably brighter red. These facts are suggestive of change in deposition and of unconformity.

Evidence of overlap was found by the writer between the Pontotoc and Enid formations, in the area south of Wildhorse Creek, in Garvin County. Successively younger beds of the Enid seem to rest on Pontotoc conglomerates, shales, and limestones on the north side of the Arbuckle anticline. Birk (FOOTNOTE 2) describes a similar unconformity around the west end of the Arbuckle Mountains.

The writer believes that the decided change in lithology, particularly the disappearance of the arkose, and the overlap, mark an important change in the sedimentary record and a pronounced unconformity which furnishes a better criterion in this area than the presence of the genus Walchia, and that the base of the Permian in south-central Oklahoma should be drawn at the base of Morgan's Asher in Pottawatomie County, and at the base of Unit 1 in Garvin County, and that no part of the Pontotoc should be included. This opinion was reached previously and independently by Birk.(FOOTNOTE 3)

Anderson (FOOTNOTE 4) disagrees with the writer's evaluation of criteria, and believes that the plant evidence is most dependable.

Morgan (FOOTNOTE 5) states:

... the selection of a contact horizon, acceptable to everyone, between the Pennsylvanian and Permian will be difficult because the transition from Pennsylvanian to Permian was very gradual. My inclination, however, is to push the contact down rather than up, and I am, therefore, more in accord with the ideas of Dr. Anderson than with yours.


In Cleveland County, Anderson found a section of rocks which could be very readily subdivided. He was able to make a classification which

FOOTNOTE 1. Robert H. Dott, op. cit.

FOOTNOTE 2. R. A. Birk, "The Extension of a Portion of the Pontotoc Series Around the Western End of the Arbuckle Mountains," Bull. Amer. Assoc. Petrol. Geol., Vol. 9, No. 6 (September, 1925), p. 989.

FOOTNOTE 3. Op. cit.

FOOTNOTE 4. G. E. Anderson, letter to C. L. Cooper, dated April 15, 1930.

FOOTNOTE 5. George D. Morgan, letter of April 23, 1930.

End_Page 124------------------------------

agrees very well with the type area at the north. As the outcrops of the formations in McClain County are very poor, and difficult to differentiate, the Cleveland County markers can not be satisfactorily traced across McClain County into Garvin County. The correlations suggested by Anderson are approximately as shown in Table II.

The thickness and general character of the Enid group in Garvin, McClain, and Cleveland counties are similar, and it seems that the character of the lithologic units composing the group should form the basis of correlations between the two areas, in the absence of traceable markers.

Since the two articles by Anderson and the writer were published, most of the differences as to correlation have been removed, and the following discussion presents the writer's present views, in which Anderson, in the main, concurs. Figure 2 shows the areal extent of the formations, as now understood, with a correction of contacts in Cleveland and McClain counties by Anderson.

Stillwater formation:
The outcrop of this formation is too far east to be present in Cleveland County. In McClain County, Anderson included in the Stillwater Morgan's Asher and the upper Konawa, making a total thickness of 390 feet.

Morgan (FOOTNOTE 1) described the Asher formation as follows.

At the bottom of this formation (Asher) is a thick series of coarse red sandstone that outcrops along the escarpment which extends form the river bridge south of Asher to the north edge of that town.


FOOTNOTE 1. George D. Morgan, "The Geology of the Stonewall Quadrangle," Bur. of Geol. Bull. 2 (1924), p. 142.

End_Page 125------------------------------

Fig. 2. Geologic map of Cleveland, McClain, and Garvin counties.

End_Page 126------------------------------

Fig. 2. Continued. See caption on page 126.

End_Page 127------------------------------

Fig. 3. Comparison of measured sections in Cleveland, McClain, and Garvin counties. Bulletin references are to Oklahoma Geological Survey publications. Approximate length of section, 36 miles. Vertical scale in feet.

End_Page 128------------------------------

The lower limit of the Asher formation probably extends south and west beyond the edge of the Stonewall quadrangle before crossing Canadian River.

The portion of the formation present in the northwest corner of the quadrangle is 250 feet thick. No upper limit is here defined.

Anderson (FOOTNOTE 1) states:

At least the upper part of the Konawa and the Asher will be considered as time equivalents of the Stillwater formation ... Unit 1 of Dott is included, and forms the top of the Stillwater formation in McClain County.

No measurements are available for the thickness of the basal Asher sandstone near Asher. Morgan evidently included some of the overlying shale in his 250 feet of section. From the height of the hill at the bridge south of Asher, it is probable that this basal sandstone has a thickness similar to that of Unit 1 in Garvin County, that is, 140 feet.

As Anderson and the writer do not agree about the base of the Permian, it follows that they disagree as to the base of the Stillwater. They agree, however, in placing the top of the Stillwater as the top of Unit 1.

Wellington formation:
Anderson (FOOTNOTE 2) makes the following statement concerning the Wellington in Cleveland County.

The Wellington consists of red, gray, and black, massive, fine-grained sandstone, with interstratified shales. In Stella township, it forms rather prominent sandstone ridges covered with timber. Small open spaces are characteristic over the shale outcrops, suggestive of park landscape... The formation grades laterally into more shale to the southeast in Pottawatomie County, on the north side of the Canadian River ... The formation has ... an approximate thickness of 400 feet.

And in McClain County:

... the Wellington changes laterally from predominating sandstone in northeastern Cleveland County to shale in southwestern Pottawatomie and eastern McClain counties. It is the opinion of the writer (Anderson) that only the lower part of Dott's Unit 2 is in reality the time equivalent of the Wellington. It ... has an approximate thickness of 100 feet. The surface outcrop in the county is characterized by gray shales, weathering into light gray soil.

Later, Anderson states:(FOOTNOTE 3)

The Wellington-Garber contact I have not yet been able to outline with any degree of certainty. This is extremely difficult on account of the changing character of the materials along the strike...

FOOTNOTE 1. G. E. Anderson, "Geology of Cleveland and McClain Counties," Oklahoma Geol. Survey Bull. 40-N (1927), p. 10.

FOOTNOTE 2. Op. cit., p. 8.

FOOTNOTE 3. Idem, letter to C. L. Cooper, dated April 15, 1930.

End_Page 129------------------------------

In Garvin County, Unit 2 is composed of shale, with thin to locally massive sandstone, essentially similar lithologically to the Wellington in McClain County and less sandy than in Cleveland County. Its thickness is 145 feet.

It is essentially a lithologic and stratigraphic unit in Garvin County, separable from the formations above and below, and, in the opinion of the writer, should be considered the equivalent of Anderson's Wellington, despite its slightly greater thickness.

Garber formation:
In Cleveland County, the Garber is mostly sandstone. The lower part, according to Anderson:

... is characterized by sandy shales, and interstratified, rather massive sandstones. In the lower 50 feet it is characterized by laminated sandstones and thin strata of pseudo-conglomerate ... Above this is a zone of barite rosettes imbedded in sandstone strata... The Hayward sandstone member (upper) is characterized by bright red sandstone strata with minor amounts of interstratified shale. The upper contact of the Garber is readily traced by its jack oak-covered surface in distinct contrast with the smoother prairie land on the Hennessey outcrop to the west ...

Barite rosettes are more abundant near the top of the sandstone, but are distributed ... through the entire Garber formation ... Pseudo-conglomerates similar to those found at the contact of Garber and Wellington are also found ... about 100 feet below the top... It has an approximate thickness in the county of 400 feet...

In McClain County:

The Garber formation is characterized by massive red sandstone lenses in the upper portion, grading into shale below... The sandstone of the Garber is covered with jack oak timber at the Canadian River, by means of which it can be traced across the Canadian River at Pecan, Cleveland County, passing 1½ miles west of Rosedale and south to the county line through the southwest corner of Sec. 31, T. 5 N., R. 1 E. The top of the Garber should correspond to the top of Unit 3 ... in Garvin County. Its thickness is approximately 200 feet.

The Garber formation is the key to the correlation of this whole series in Garvin County with the beds in the counties at the north. The greatest thickness of predominating sandstone beds is found in Units 3 to 6, inclusive, and it is this group which the writer now considers to be equivalent to the Garber sandstone. There is a striking similarity in lithology, sequence of beds, and thickness between these rocks and Anderson's Garber in Cleveland County.

The base of Unit 3 is marked by a prominent bed of pseudo-conglomerate overlain by barite rosettes. These are particularly well developed in the vicinity of Whitebead and in the north part of T. 4 N., R.

End_Page 130------------------------------

1 E., near the county line. This occurrence, so similar to that described by Anderson at the base of the Garber in Cleveland County, leaves little question in placing the Garber-Wellington contact at the top of Unit 2.

Pseudo-conglomerates occur at the middle and top of Unit 3, and barite rosettes occur at several horizons. Pseudo-conglomerate was found also in the lower part of Unit 5, and barite rosettes near the middle of Unit 6. The sandstones of this series (Units 3-6) are mainly bright red, in contrast to those of Unit 1, which are mainly red-brown. This group was called "red sandstones" by Miser.(FOOTNOTE 1) Unit 6 is the top of the sandstone series, and has been eroded into rough topography, and is largely timber-covered. Its outcrop is in decided contrast to the prairie land on the west.

Attempts to trace the Garber-Hennessey contact across McClain County were attended with disappointment, as the Garber is mostly shale, or is covered by windblown sand from Washita River. Just east of the town of Paoli is an outcrop of massive sandstone which could easily be considered the top of the Garber, and was so considered by Anderson at the time of writing his report on Cleveland and McClain counties.

After studying the area south of Washita River, and comparing the entire section in that area with Anderson's section in Cleveland County, the writer was forced to the conclusion that the Garber-Hennessey contact must be considerably higher, and that the bed at Paoli was more nearly the top of Unit 3. Thus, the contact would trend southwest from a point 1½ miles west of Rosedale, to Sec. 19, T. 5 N., R. 1 E., thence west and south nearly to the middle of the north line of T. 4 N., R. 2 W. It may, therefore, be correlated with the top of Unit 6, south of Washita River. The trace of this contact is essentially parallel with that of the next higher Duncan-Hennessey contact.

Anderson recently stated:(FOOTNOTE 2)

I believe Dott is approximately correct in his tracing of the Garber-Hennessey contact through T. 4 and 5 N.

Such a correlation would add approximately 200 feet to Anderson's Garber in McClain County, giving a thickness of approximately 400 feet.

Hennessey formation:
In Cleveland County, the Hennessey shale (FOOTNOTE 3)

... is characterized by predominating red shale, thin and frequently laminated... Its lower 50 feet contain several thin, rather resistant sandstone

FOOTNOTE 1. Hugh D. Miser, "Geologic Map of Oklahoma," U. S. Geol. Survey (1926).

FOOTNOTE 2. G. E. Anderson, letter to C. L. Cooper, dated April 5, 1930.

FOOTNOTE 3. G. E. Anderson, "Geology of Cleveland and McClain Counties," Oklahoma Geol. Survey Bull. 40-N (1927), p. 9.

End_Page 131------------------------------

strata which form characteristic flat-topped hills along the east border of the prairie... It has an approximate thickness of 600 feet in the county.

... It is predominantly shale in McClain County, with a few sandstone lenses which rapidly grade laterally into shale ... The most notable of these is the sandstone bluff on the south side of the Canadian River, immediately north of Purcell. Another such sandstone lens forms a rather high bluff in Sec. 21, T. 8 N., R. 2 W., on the north side of the Canadian, and still another, though less prominent, two miles west of Purcell, on the north line of Sec. 16, T. 6 N., R. 2 W., all these apparently at different stratigraphic horizons within the shale ... The Hennessey forms a broad belt, many miles wide, and has a uniform thickness of approximately 650 feet.

As the Garber-Hennessey contact in McClain County is now thought to be west of, and stratigraphically higher than, the contact mapped by Anderson, the thickness of the Hennessey would be less, probably about 450 feet.

In Garvin County, Units 7 and 8 are predominantly shale, with several sandstone horizons, particularly at the base and middle of Unit 7, at the base and top of Unit 8. Those at the base of Unit 8 are persistent, and can be traced nearly across the county. They strongly resemble the sandstones mentioned by Anderson as occurring at the base of the Hennessey in McClain and Cleveland counties. They probably could be recognized north of Washita River, in McClain County, and may lie at the horizon of one of the sandstone beds mentioned by Anderson. Thin, white beds of sandstone occur in the red shale, near the top of Unit 8, giving a banded appearance. Frank C. Greene suggests that these may lie at the approximate horizon of the Merkel dolomite of north-central Texas.

The topography is that of gently rolling prairie land. The thickness of the two units is about 430 feet. Table I of Oklahoma Geological Survey Bulletin 40-K (FOOTNOTE 1) is in error as to the thickness of Unit 8. It should read 300 feet instead of 500 feet.

The lithologic character, gently rolling topography, vegetation, and the stratigraphic position between two massive sandstones leave little doubt as to the equivalence of Units 7 and 8 with Anderson's Hennessey.


The writer proposes to subdivide and correlate the Red-beds in Garvin County which lie between the Pontotoc formation and the Duncan sandstone, shown in Table III.

FOOTNOTE 1. Robert H. Dott, "Geology of Garvin County," Oklahoma Geol. Survey Bull. 40-K (1927), p. 20.

End_Page 132------------------------------


SHERWOOD BUCKSTAFF, Pauls Valley, Oklahoma (written discussion received, July 11, 1931):
Mr. Dott's revision of earlier correlations in Garvin and adjacent counties is another step toward the solution of early Permian stratigraphy in southern Oklahoma. The writer, however, considers that one of his correlations, that of the Garber as equivalent to Units 3 to 6, is not proved. The correlation, as he states, is based largely on thicknesses and lithologic similarities. Lithologic similarity of red beds, deposited under rapidly changing conditions of sedimentation, is a dangerous criterion for correlation. The sequence of sandstones, shales, pseudo-conglomerates, and barite rosettes is, in the writer's opinion, merely a repetition, in two areas, of sediments commonly occurring and not a proof of simultaneity. The structural evidence is not compatible with this correlation. Mr. D tt's Garber-Hennessey contact across T. 5 N. is obviously arbitrary, connecting two areas of sandstone; but it is at variance with the true strike of the formations as revealed by detailed mapping in the west half of T. 5 N., R. 1 E. and the southern part of T. 5 N., R. 1 W. Furthermore, it transgresses the strata from the top of Unit 3, in the northwest part of T. 5 N., R. 1 E., to the top of Unit 6 in the center of T. 4 N., R. 2 W. The parallelism between this contact and the overlying Duncan-Hennessey contact, remarked by Mr. Dott, is not a true structural parallelism. The Duncan-Hennessey contact, across the area involved, becomes progressively lower topographically as it is traced southwestward into the Washita River valley, and the strike of the contact is more east of north than t e strike of the beds. The Garber-Hennessey contact as shown by Mr. Dott's map rises from the South Canadian River Valley to the top of the topography and drops into the Washita Valley unaffected by the topography. If this contact were traced to conform with the topography, the parallelism of the two contacts would vanish.

As Mr. Dott notes, the "Garber" in T. 5 N., R. 1 W. is largely shale. The top of the sandstones of Unit 3, south of South Canadian River, seems to be


End_Page 133------------------------------

100 feet or more lower than the top of the accepted Garber north of the river. Evidences of structural irregularity being lacking across the river, this discrepancy must be due to gradation of the upper sandstones of the Garber to shale. Such a gradation is in accord with the greater thickness, greater sand content, and closer proximity to the source (not necessarily the ultimate source, but the locus of maximum deposition of sediments) of the Garber farther north. Mr. Dott assumes that as the shales grade back to sandstones, south of Washita River, the tops of the two sandstone zones are equivalent. There is no logical basis for this assumption, as the sandstones south of Washita River, becoming laterally more shaly northward and more sandy southward, almost certainly come from some ource southeast of the area, and are not necessarily contemporaneous with the Garber sandstones. Detailed mapping of zones and thicknesses indicates that the top of the sandstone south of Washita River (top of Unit 6) is approximately 160 feet higher stratigraphically than the top of the Garber north of South Canadian River. Consequently, the writer considers that the true Garber-Hennessey is not as shown by Mr. Dott's map, and, by the nature of the sedimentation, can not be traced by lithology across Garvin County.

ROBERT H. DOTT, Tulsa, Oklahoma (written discussion received July 13, 1931):
An acceptance of any opinion relative to red-bed stratigraphy in south-central Oklahoma must depend on an agreement as to criteria.

All workers would accept the absolute and unquestioned tracing of some key bed. If the top of the Garber as defined in Cleveland County could be definitely traced across McClain County into the units of Garvin County, the stratigraphic position of that horizon in the Garvin County units could be established to the satisfaction of everyone.

The broad Washita valley in northern Garvin County alone makes such tracing impossible. Lacking such irrefutable evidence, other criteria must be applied, and stratigraphic sequence, lithologic similarity, and similar thicknesses seem to be of as great value in this type of beds as any other. Certainly it can not be said that they fail in the area north of Canadian River, between Cleveland and Oklahoma counties, where actual tracing can be done.

From Purcell to Oklahoma City a thick, dominantly sandstone series, cropping out in a terrane of rugged, jack oak-covered hills, is overlain by a likewise thick shale, whose outcrop is characteristically treeless, and in whose basal 100 feet occur several persistent sandstone ledges. These criteria are referred to repeatedly as marking the Garber-Hennessey contact in those counties. If they hold for a distance of 40 miles along the strike, is it unreasonable to assume that an almost identical sequence in Garvin County, 12 miles farther south, marks the same contact?

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