USA > Iowa > Chickasaw County > History of Chickasaw and Howard counties, Iowa, Volume I > Part 6
USA > Iowa > Howard County > History of Chickasaw and Howard counties, Iowa, Volume I > Part 6
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Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14 | Part 15 | Part 16 | Part 17 | Part 18 | Part 19 | Part 20 | Part 21 | Part 22 | Part 23 | Part 24 | Part 25 | Part 26 | Part 27 | Part 28 | Part 29 | Part 30 | Part 31 | Part 32 | Part 33 | Part 34 | Part 35 | Part 36 | Part 37 | Part 38 | Part 39 | Part 40 | Part 41 | Part 42 | Part 43
DEVONIAN SYSTEM
GENERAL DESCRIPTION
The Devonian rocks of Howard County are all calcareous; all are more or less magnesian; the greater portion of the entire system would be classed as impure dolomite. True shales were not observed anywhere. Dolomitization of the Devonian is more common in the northern part of the state than at the southern outcrops in Johnson, Cedar, Muscatine and Scott counties, thus revers- ing the rule that has been observed in relation to the dolomitization of the Galena-Trenton. The lowest beds seen in Howard County belong to a horizon far above what has been recognized as the base of the Devonian in the southern part of the area of its distribution. The beds which rest directly on the Maquo- keta contain Stropheodonta demissa Conrad, Productella subalata Hall, Atrypa reticularis Lin., Atrypa aspera Schlot., Spirifer pennatus Owen and Cyrtina ham- iltonensis Hall. The fauna indicates a horizon equivalent to that represented about the middle of the quarries at Independence in Buchanan County. In this zone in Howard County, Productella is the most abundant and most characteristic fossil, and it is convenient to refer to the horizon as the Productella beds. This zone belongs to the Upper Davenport beds of Norton, below which, before reaching the base of the Devonian in Linn, Cedar and Scott counties, there are the diversion of the Wapsipinicon stage which have been described as Lower Davenport, Independence, Otis and Coggan.
There are here some interesting and puzzling anomalies in the distribution and vertical range of certain species, which are deserving of notice. For example, the Productella beds have a thickness of forty feet, a thickness more than twice as great as that of the corresponding beds at Independence. They
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are overlain by fifteen to twenty feet of coarse dolomite characterized by the inclusion of large masses of crystalline calcite. In these coarse, calcite-bearing beds there are occasional casts and impressions of Favosites alpenensis and Acer- vularia davidsoni. These corals are in their usual stratigraphic relation to Pro- ductella, and so far the succession of life zones is in accord with the Devonian section in Buchanan County. But in the Salisbury quarry at Vernon Springs, twenty feet or more above the top of the coarse, coral-bearing dolomite, there are layers only slightly magnesian in which Gypidula comis, Atrypa aspera, and the lenticular, elongated, finely striated type of Atrypa reticularis, known here- tofore only from the horizon of the Independence quarries, are well preserved. This particular form of the Atrypa reticularis should be found below the coral horizon and never above. Its place is with Productella. At Independence the Gypidula is found, rather sparingly, as high as the beds carrying Productella, but from Independence to Davenport, Gypidula is more characteristic of the Lower Davenport beds than of any other horizon, and yet the Lower Davenport beds are not even represented in Howard County. These forms seem to have remi- grated into this territory long after they had permanently disappeared from other parts of Iowa.
Typical Exposures. I. The lowest member of the Devonian section in How- ard County, composed of the Productella beds, is typically exposed at the bridge over the Turkey River on the Howard-Winneshiek county line. At the level of the road, at the south end of the bridge, the deposit is soft, yellow, earthy dolomite which is broken into irregular nodules as a result of weathering. The fossils occur only as casts or impressions, but it is possible to recognize Strophe- odonta demissa, Productella subalata, Spirifer pennatus and Cyrtina hamiltonen- sis. Besides these there are casts of small undetermined gastropods and pygidia of Phacops. It is about twenty-five feet from the level of the bridge down to the water in the river. The slope is covered with waste, but 150 yards west of the bridge the wash of a small intermittent stream exposes the beds to the level of the narrow flood plain. With the exception of one or two layers that have been quarried on a small scale, the rock is soft and easily disintegrated into a yellow sand or marl. The harder layers, which occur about the middle of the section, contain indistinct impressions of a small shell-like Spirifer subum- bonus Hall. The Productella beds are well shown in the river bluffs at a number of points in section I, New Oregon Township. In the northeast quarter of the section there are massive, undecayed ledges of the Productella horizon, forty feet in thickness.
Along the east side of the northeast quarter of section 24, Vernon Springs Township, there are exposures of badly broken and weathered limestone, soft and magnesian, but rich in Productella and the forms usually associated with it. The full thickness of this part of the Devonian column, about forty feet, is indicated by the rather unsatisfactory outcrops on the long sloping hillside. Near the summit of the hill the next higher member of the series is seen, but after passing the crest the Productella beds reappear on the slope descending to the Valley of Silver Creek. These beds are again seen north of the southeast corner of section I of the same township. In Albion Township the exposures of the Productella horizon are quite numerous, though they are rather unimpor- tant and unsatisfactory. In the northeast quarter of section 36 a small quarry
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has been worked at this horizon. The beds are also exposed in the northwest quarter of section 27, at a point one-fourth of a mile north of the center of 22, and at numerous other small breaks and outcrops along the Devonian margin, in the northern part of the township.
The most important exposure of the Productella beds occurs at Foreston, near the northwest corner of section 14, Forest City Township. As usual in this part of Iowa, the rock is a rough, vesicular dolomite, rather soft and non- crystalline. The bedding planes are largely obliterated, and the fossils occur only as casts. The exposed section is made up of a number of heavy ledges, all very much alike. The beds have been quarried quite extensively, the massive blocks being used in the construction of the mill dam and in other structures where weight and strength are the most desirable characteristics. At the south end of the quarry is shown the massive character of the layers and the rough vesicular appearance of the freshly broken surfaces. At the north end of the quarry the following section was noted :
Feet
5. Decayed ledges badly broken up and divided into compara- tively small blocks. 8
4. Coarse, vesicular, undecayed bed, very fossiliferous, casts and impressions of Productella subalata common, impres- sions showing the coarse ribs and strong spines of Atrypa aspera numerous, pygidium of Phacops seen occasionally. 5
3. Coarse, pitted layer like No. 4, with many casts of brachio- pods among which Productella is the most common. 41/3
2. Soft, light yellow bed with casts of Atrypa reticularis. 3
I. Bed like No. 2, but softer and more granular, with few fos- sils, mostly Atrypa reticularis, bed divides in places into four parts each about one foot in thickness, in places the parts are fused together on account of the complete oblit- eration of the bedding planes 4
There are massive ledges of the Productella beds in the steep bluffs facing the river in section 12 of Forest City Township and section 7 of Albion. The lower part of the bluffs, for thirty or forty feet, is occupied by the upper portion of the Maquoketa formation, the heavy beds of the Devonian appearing in some places quite conspicuously above the Maquoketa, well up on the steep hillsides. It is the Productella beds that are seen at the level of the water below the mill, at the old Town of Lime Springs. Above this point the dip of these beds soon carries them below the level of the stream.
2. The member of the Devonian series which follows the Productella beds in Howard County is the equivalent of the Acervularia davidsoni beds of Buchanan County. It is made up of a succession of coarse, dolomitic layers ranging from a few inches to more than a foot in thickness. A typical exposure of these layers occurred on the north side of the stream, immediately below the mill dam at Vernon Springs ; and all the way to the east line of the county these beds may be seen in the bluffs of the Turkey River, overlying the Pro- ductella horizon. As the county line is approached they are found to occupy a
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position forty feet or more above the level of the stream. Lithologically, these beds resemble certain phases of the Niagara limestone in Delaware and Dubuque counties, except that, in place of the chert usually found in the Niagara, there are large included masses of calcite. This calcite differs from that which will pres- sently be described as lining spherical or definitely shaped cavities in beds higher up in the series. The spaces it occupies are shapeless and irregular and are completely filled. The formless, cleavable masses are devoid of any indications of crystal faces or crystal outlines.
The exposure at the mill dam covers a comparatively large area and gives an unusually favorable opportunity for the study of the beds in detail. Besides the characteristics already noted, this horizon is distinguished by the presence of casts of Favosites alpenensis and Acervularia davidsoni. The presence of the corals and the stratigraphic position of the beds both lead to a correlation of the horizon with the Acervularia davidsoni zone at Independence, Littleton, Waterloo and Iowa City.8 The marked differences in the texture and composi- tion of the rocks and in the perfection and abundance of the fossils are due in part at least to the great changes which were wrought during the process of dolomitization. These coarse dolomitic beds with their shapeless masses of calcite are seen at intervals above the Productella horizon, along the east line of the county from section 25, Vernon Springs Township, to the north line of section 36 in Albion. They may be recognized, over and over again, in their proper relations, all around the Devonian margin. One of the most fossiliferous expo- sures of this phase occurs in the side of a ravine near the middle of the west line of section 15, Albion Township. The common Favosites alpenensis is comparatively abundant. Beds belonging to essentially the same horizon are found in the Croft quarry at Elma, in section 1, south of the middle of Afton Town- ship. At the bottom of the quarry there is a dark brownish, crystalline, dolo- mitic layer which in general forms the floor. It has, however, been taken out over a few square yards; it is very fossiliferous, but the fossils occur only as casts. The forms recognized are Favosites alpenensis, Stropheodonta demissa, Pentamerella dubia, Atrypa reticularis, Spirifer subvaricosus, S. asper, S. fim- briatus, a large species of Gomphoceras, and a small species like G. oviforme. This fauna belongs to a horizon just below the Acervularia zone, and its equiv- alent in the northeastern part of the county should be included in the lower part of the coarse calcite-bearing beds.
. 3. South of the bridge at New Oregon, above the calcite-bearing beds described in the foregoing paragraphs, there are twenty feet of variable strata, composed in part of soft, earthy limestone grading into marly shales, and in part of fine- grained, whitish, non-dolomitic limestone. The section is not very satisfactory. In fact the beds of this horizon were not well shown at any point in the county. The non-dolomitic phase of this member of the series is seen in loose. weath- ered, crackled blocks, a short distance west of the middle of section 24. Vernon Springs Township. A better exposure of the crackled beds occurs about eighty rods south of the northeast corner of section 10, and a still better illustration of this special phase is found in the northeast quarter of section 12, all in Vernon
8 Compare the "Coral Reef Bed" in the report on Johnson County, Iowa Geol. Surv., Vol. VII, and the "Acervularia Zone" in the report on Buchanan County, Vol. VIII.
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Springs Township. In section 12 the beds are rich in stromatoporoids similar to the forms occurring in the stromatoporoid reef from Mitchell, Worth and Cerro Gordo counties on the north, to Johnson County in the southern part of the Devonian area. Besides the stromatoporoids, the beds carry a small digitate Favosites and the usual gastropod of this horizon, Euomphalus cyclostomus. The non-dolomitic, fine-grained, white limestone of this horizon is the equivalent of the beds described as "fine-grained, white limestone" in the reports on John- son and Cerro Gordo counties. This phase of the Devonian, which is always associated with the stromatoporoid horizon, attains its fullest development in Mitchell County and in the northern part of Floyd, where it takes on the char- acteristics of a fine lithographic stone. The same lithographic phase, but less perfectly developed, occurs at LeRoy in Minnesota, a short distance from the north Howard County line. This third member of the Devonian series is quite variable. While in the northeastern part of the county the non-dolomitic stromat- oporoid beds occur in it, these beds are not always present. The greater part of this portion of the section is a soft, magnesian, earthy limestone, which breaks down rapidly into a marly clay or into irregular concretionary fragments. The exact line separating this from the next overlying member of the section could not be definitely traced.
4. The beds which follow No. 3 in ascending order are typically represented in the quarries at Vernon Springs. One of these quarries, which was formerly worked quite extensively, is located on land belonging to H. C. Salisbury, in the southwest quarter of the southwest quarter of section 34, Vernon Springs Township. Other exposures occur in the Patterson quarries in the northeast quarter of the same quarter section, and in a small quarry near the river in the southwest quarter of section 33. At the base of the Salisbury quarry there are several courses of firm bluish limestone not dolomitic. The individual courses are from one to two feet in thickness, and the aggregate exposed is about eight feet. The fossils are mostly brachiopods and the shells are well preserved. Among the species noted are Gypidula comis, Atrypa aspera and the fine lined type of Atrypa reticularis found in the quarries at Independence. Reference has already been made to the fact that this fauna seems very much out of place in a position above the Acervularia and stromatoporoid horizons. The concur- rence of these special types of brachiopods is unknown elsewhere 9 except at
9 The three species, Gypidula comis, Atrypa aspera and Atrypa reticularis, occur together in the Lime Creek shales at Rockford in Floyd County and at Hackberry Grove in Cerro Gordo, but in all three cases the forms are varietally different from those at Independence. The A. aspera at Independence is Hall's variety A. occidentalis; while the similar species in the Lime Creek shales has been referred to the variety A. hystrix. The A. reticularis of the two horizons differs very strikingly in size, markings and general propor- tions, and the Gypidulas are sufficiently distinct to make their separation a simple and easy matter. The species as they occur in the Salisbury quarry are all of the types found at Independence. These species all persisted somewhere-in the meantime suffering more or less modification in form-during the interval which separated the age of the quarry stone at Independence from that of the shales at Rockford, and the fact that they migrated into Iowa and temporarily occupied some parts of it at different times during the interval, need occasion no surprise. The re-migration which enabled them to occupy Howard County long after they had disappeared from Buchanan, occurred before modification had progressed to any appreciable extent.
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the horizon of the quarry stone at Independence, a horizon which corresponds to that of the Productella beds of Howard County.
5. In the Salisbury quarry the beds last described are overlain by soft, gran- ular, magnesian limestone stained more or less with iron oxide and varying in color from dirty yellow to dull brown and red. This fifth division of the Howard County Devonian has a total thickness of at least fifty feet. It furnishes the best of the building stone quarried in the county. It is the equivalent of the "Yellow, earthy limestone" quarried near Littleton and described in the report on Buchanan County (Iowa Geological Survey, volume VIII, page 234). Lithologically the beds are very similar in Howard and Buchanan. The characteristics are unusually persistent.
The Salisbury quarry affords the following section :
Feet
5. Black soil mixed with broken rock. I
4. Rock in broken, angular fragments affording an illustration of how the stone yields to frost and weather. 4
3. Heavy courses of good building stone, soft, magnesian, yellow or brown in color, containing numerous spheroidal cavities lined with crystals of calcite, fossils rare and represented only by casts 8
2. Band of softer, more argillaceous limestone in three or four layers, calcite lined cavities numerous. 3
I. Courses of more solid and purer limestone from one to three feet in thickness, fossil shells preserved. 7
Number I of this section is composed of the beds already described, which constitute the fourth member of the Devonian series, while 2, 3 and 4 represent the lower part of the fifth. All the beds of the quarry are cut at short intervals by oblique joints. The other quarries in the immediate vicinity of Vernon Springs show nothing essentially different from what is seen in the quarry described. In the northwest quarter of section 33, Vernon Springs Township, quite an amount of stone has been taken out, and the opening shows three heavy ledges, each about three and a half feet in thickness, cut by numerous joints, and presenting many vug-like, or geode-like cavities lined with calcite. The rock resembles No. 3 of the Salisbury quarry, but the beds are higher in the series. The workable layers are overlain by from four to five feet of small, angular, worthless fragments which have resulted from the disintegration of still higher beds.
The largest quarry in the county is operated by John Hallman near the Fair Ground, in the western edge of Cresco. It has been opened by working down beneath the surface of the level prairie. In stratigraphic position the beds here lie above any heretofore noted and are probably the highest to be found within our territory. The rock is earthy, magnesian, rather soft, but it seems to be capable of standing the weather fairly well. At some points the quarry has been worked to a depth of twenty feet. Toward the top the bedding is quite regular in places for a thickness of eight feet, and the stone may be taken out in courses ranging from three to six inches in thickness. The whole deposit
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is very irregularly jointed, the joints cutting the beds at every angle from vertical- to horizontal. In the lower part of the quarry the bedding is quite irregular, the courses are thicker and they pitch and roll in the most confused way, in different directions. Crushing and movement since the deposit was laid down are indicated by the general development of slickensides on the joint faces. Fossils are very rare. A few impressions of what seemed to be Stropheodonta demissa were noted, together with obscure fragments of plates of fishes.
The upper part of the Salisbury quarry and the higher beds exposed in the other openings near Vernon Springs are represented in a small opening from which a considerable quantity of good building stone has been taken, in the northeast quarter of the southeast quarter of section 14, Forest City Township. The location is on one of the high points in an area of thin drift and consequent preglacial topography. There is not more than six inches of soil above the four- foot band of decayed and broken stone which represents the effects of frost and weather. Below the fragmentary band the stone is sound, lies in heavy ledges, is freer than usual from calcite lined cavities and is capable of affording dimension blocks of fair sizes. Much of it is streaked with iron oxide, a fea- ture, however, better shown in the next quarry to be described. The only fossil observed here was an imperfect impression of a closely coiled, nautiloid sephalo- pod. The same beds are shown in a somewhat extensively worked quarry belonging to M. H. Jones, in the southeast quarter of the southeast quarter of section 24, Chester Township. The beds are soft, granular and magnesian, as usual. They are stained by the secondary infiltration of iron oxide which is arranged in flexuous, concentric, parallel bends around certain nuclei, the dispo- sition of the bands being in no way influenced by joints or lamination planes. The vug-like cavities lined with calcite are common. This completes the obser- vations made on the fifth member of the Devonian column. The beds of the Jones quarry are not as high in the series as those in the Hallman quarry at Cresco. The total thickness of this member is at least fifty feet.
In the river valley above the old Town of Lime Springs, it is the members immediately overlying the Productella beds that first appear. The dip, however, is very slight in this direction. the river valley being almost parallel with the line of strike. At Glen Roy mills in section 19 of Forest City Township, only about three-fourths of a mile northeast from the Jones quarry, there is an exposure in the river bank, of non-dolomitic, shaly, nodular limestone overlying some soft, yellow, marly beds, all of which belong to division 3 of the Devonian column. On higher ground in the southwest quarter of section 18 there are the beds which lie at the base of the Salisbury quarry, the beds which have been described as the fourth member of the column. All the species enumerated from that member are found here, and there is here the additional species, Orthis iowensis. The Jones quarry beds overlie these last and represent the uppermost member of the Howard County Devonian.
Between the mill and the bridge at Chester there is an exposure of soft magnesian limestone stained with concentric streaks of iron oxide, as are the beds of the Jones quarry. The horizon, however, is near the base of the second division of the Howard County Devonian. Casts of fossils are more than usually common, and among the recognizable species are Atrypa reticularis, Spirifer
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subvaricosus and S. fimbriatus. The same spirifers occur elsewhere in Iowa, in the Cedar Valley stage of the Devonian, a few feet below the Acervularia beds and not far above the Productella horizon. Their position at Chester is the normal one. At Le Roy, in Minnesota, some distance farther up the river, it is the lithographic phase of number 3 that is most conspicuous in the small local quarries.
Under 2 on page 55, reference is made to the Croft quarry near Elma. The fauna enumerated from the lower part of this quarry corresponds to that found near the bridge at Chester, and, notwithstanding some lithological differences, the geological position is the same. The Croft quarry lies west of the railway : another quarry is located one fourth of a mile farther east. All the beds in both quarries may be referred to the horizon of the coarse, calcite-bearing member, number 2, exposed at Vernon Springs. No fossils were seen above the basal layer of the Croft quarry. The overlying limestone is regularly bedded, coarse-grained, contains large amounts of calcite, lies in layers ten inches to a foot in thickness at the bottom, but toward the top of the quarry splits into thin flags two or three inches in thickness.
Beds corresponding to the upper part of the Croft quarry have been worked for building stone at points from three and a half to four miles west of Elma. There is one opening on land of M. Monaghan near the center of section 8, and another on land of J. Roche in the western edge of section 9, in the southern part of Afton Township. Both of these quarries were opened in the surface of the level prairie. Neither has been operated for a number of years. Soil has washed down over the face of the layers, and growth of vegetation has helped to obscure the situation. There is another abandoned quarry three-fourths of a mile south of Elma, on land belonging to Henry Miller. A rank growth of weeds and bushes conceals all the layers except one heavy, dolomitic ledge eighteen inches in thickness. This point is higher than the Croft quarry, and the beds are probably equivalent to some part of the third division of the Howard County Devonian.
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