USA > Iowa > Decatur County > History of Decatur County, Iowa, and its people, Volume I > Part 24
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3. Shale, gray to drab. 4
2. Shale, black "slate" 1
1. Shale, gray, sandy 6
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HISTORY OF DECATUR COUNTY
The limestone found on Weldon River (southeast section 15, Morgan Township) is probably also the Earlham. The only fossils collected were Athyris subtilita and Archæocidaris. The outcrops indicate that higher limestones occur.
In the western portion of the county there are a number of excel- lent exposures of the various members of the formation. Many of them will be referred to in the notes on the quarries. The exposures in the eastern portion of the county are rare and with the thick drift present it is difficult exactly to locate the eastern limits of the forma- tion. As laid down upon the accompanying map the line is subject to some correction. The limit in the southeastern corner of the county is probably quite correct, though there may be an outlier east of Caleb Creek. Farther north it is fixed by some exposures on White- breast Creek in Clarke County. Between these points it may be found to extend a little farther to the east or west than is indicated.
PLEISTOCENE
In recent years the unconsolidated materials which so generally form the surface formations have attracted considerable attention. This is particularly true of those beds which were laid down by, or in connection with, the great glaciers or ice sheets which, in the period immediately preceding historic times, spread over much of North America as well as certain portions of the Old World. The deposits made by the ice sheets are well displayed in Iowa and have been found to be of peculiar interest. Within the last year or two it has been shown that the drift deposits of this state have had a much more con- plex history than has been heretofore ascribed to them. Near Afton in Union County to the north, and again in Harrison County, Mo., to the south, certain phenomena of more than local interest have been observed. When the study of Decatur County was taken up it was hoped that in the exposures along its deep cut valleys decisive evi- dence on certain mooted questions would be obtained. The result of the investigations are neither altogether satisfactory or altogether disappointing. Their value and bearing upon general questions may, however, be better estimated after a review of the evidence.
The drift deposits of Decatur County include the Kansan bowlder clay, with certain possibly older beds, the gumbo, the loess and the alluvium. The latter is the most recent deposit and is found along all the streams, occupying the lowlands. The loess is the surface formation over the upland and runs over the divides and down into
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HISTORY OF DECATUR COUNTY
the valleys in the form of a mantle. The gumbo is under it and has the same stratigraphic relations as the loess. The drift deposits proper are under the gumbo and often under the alluvium. They cover the whole of the upland region to a variable depth, averaging probably 150 to 200 feet. The drift also runs down into the pre- glacial valleys.
KANSAN AND OLDER DRIFT SHEETS
The drift sheet left by the major advance of the Keewatin ice sheet and extending out from under the later Iowan and Wisconsin tills is known as the Kansan drift. It is believed to have extended on the south to the Missouri River and on the southwest across that stream into Kansas. When named 27 it was thought to be the oldest drift sheet in North America. Dawson 2$ has since shown that in Canada there is an older drift, named by him the Albertan, and the evidence of two drifts in Southern Iowa, long since noted by Cham- berlin 2? and McGee, has been interpreted as indicating a pre-Kansan drift 30 in that region.
The interpretation accords with the results obtained from a study of the Alps 31 to the extent that it postulates two old drift sheets. In the latter region there is, outside the moraine of the last glacial period, evidence of two older and widely separated invasions of the ice, the younger of the two apparently representing our Kansan. The inter- pretation here offered is also in harmony with numerous other phe- nomena. In a word it may be stated that under the Kansan drift there are traces of a still older drift, though the limits of this older drift are not known, nor is the evidence with regard to its existence everywhere as satisfactory as could be desired.
The surface drift throughout Decatur County is old. This is shown not only in the topography, but in the condition of the drift itself. Where the surface of the bowlder clay has not suffered recent
27 Chamberlin: Gekie's "Great Ice Age," pp. 773-774. 1894. Jour. Geol., Vol. III, pp. 270-277. 1895.
28 Dawson: Jour. Geology, Vol. III, pp. 507-511. 1895.
29 Chamberlin: Loe. Cit. McGee: Pleistocene Hist. N. E. Iowa, Eleventh Ann. Rep. U. S. Geol. Surv., pp. 493-499. 1891.
30 Chamberlin: Jour. Geol., Vol. IV, pp. 872-876. 1896. Calvin: "Annals of Iowa" (3), Vol. III, No. 1, pp. 1-22. 1897. Iowa Geol. Surv., Vol. VII, pp. 18, 19. 1897. Amer. Geol., Vol. XIX, pp. 270-272. 1897. Bain: Trans. Iowa Hort. Soc. 1896. Iowa Geol. Surv., Vol. VI, pp. 463-467. 1897. Ibid., Vol. VII, pp. 335-338. 1897.
31 See "Le Systems Glaciaire des Alpes, guide publie, a de occasion du Congres geologique International, 6 m Session, Zurich, 1894, par M M. Penck, Brückner et du Pasquier. (With references.) .
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HISTORY OF DECATUR COUNTY
erosion it is uniformily highly colored. The iron content has been oxidized until a reddish-brown surface corresponding to the "fer- retto" of Italian geologists has been produced. This reddish-brown grades through orange to yellow below, and the yellow in turn gives place to blue, which is the fundamental color of the Kansan bowlder clay. Often the yellow is seen following down into the blue along cracks and fading out from their edges. All the evidences indicate that here, as elsewhere, the blue and yellow clays belong together. The change in color is a matter of oxidation, and is most marked when the oxidation has been most active.
The blue bowlder elay and much of the yellow contains a large amount of calcium carbonate, fine limestone dust. This causes it to give a vigorous reaction when tested with acid. The upper surface of the bowlder clay gives no reaction, and the strength of the reac- tion, increasing from nothing at the surface to full vigor at a depthi of 7 to 9 feet, is proportional to the amount of leaching which the clay has suffered, which in turn is approximately proportional to the depth below the surface. The bowlder clay contains a con- siderable variety of pebbles and bowlders, they being in most cases flattened and planed, and often showing striations.
In a eut on the Humeston & Shenandoah Railway, near De Kalb, the following kinds of rock were observed in the till: Gray and red granite, red porphyry, Sioux quartzite coarse and fine-grained, quartzite with pebbles of clean quartz and red jasper, gabbro, fine- grained greenstones, iron concretions, bits of clear, white quartz, small pieces of limestone, chert and very small bits of sandstone. The sandstone and limestone doubtless come from the coal measures of the adjacent region. The quartzite, including that with the quartz and jasper pebbles, probably came from the Sioux Falls region. The granites and greenstones came from farther north. Many of the granite cobbles, both large and small, are so badly weathered that they may be easily picked to pieces with the fingers. This is par- ticularly true of those near the top of the formation and becomes less noticeable toward the bottom. It is not confined to particular kinds of rock which might be supposed to weather easily, such, for example, as coarse-grained granites with large feldspars, but is true of a wide variety of stones.
It is believed that the weathering of the granites, the oxidation of the iron and the decalcification of the bowlder clay, in view of their obvious relationships to the original surface of the latter, are to be interpreted as evidence of a long period of subærial decay after the
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HISTORY OF DECATUR COUNTY
bowlder clay was deposited. The ferretto surface maintains itself under the loess and outlines the present topography, so that this period of exposure, which the advanced stage of the topography indi- cates, must have been a long one, occurred after the bowlder clay was laid down, and before the overlying gumbo and loess were deposited. It is this drift which forms the bulk of the Pleistocene deposits of the county and which has been called the Kansan. Relative to the ques- tion of a possible pre-Kansan there are certain exposures of interest.
In section 36, Pleasant Township of Union County, the following exposure is seen in the bank of Grand River near the ford. This is within a mile of the northwest corner of Decatur County:
FEET
4. Loess-like top soil. 1
3. Sand, fine to coarse, with some gravel below 6
2. Gravel, sandy, much weathered material: 10
1. Bowlder clay, blue-black, in physical character resembling the older bowlder clay at Afton Junction 12
The sand and gravel are evidently waterlaid beds and belong together. They graduate laterally into a reddish clay and these into a drab to blue bowlder elay. This shading off of the gravels into a bowlder clay is true as well of the gravels at Afton Junction. The gravel found at this exposure is similar in every regard to that found farther up the river. It wants only the bowlder elay over the gravel to make the exposure complete, and as the exposure is some distance below the high land, there can be little doubt of a higher bowlder clay. Between the two exposures there are traces of the same beds, and it is evident that what explains one exposure must serve also to explain the other.
About three miles northwest of Davis City (center of section 28, Burrell Township) a bluff at another ford across Grand River shows an interesting drift exposure. The hills here on the south are close to the river. A spur runs out a little from the bluffs, as indicated on the sketch map.
The nose of this spur has been cut across by the river, making the exposure. At the water's edge stratified sands are exposed. Fifteen feet above the water is a well marked soil horizon buried beneath thirty feet of yellow bowlder elay sloping up to the bluff 150 feet high. The bowlder clay is evidently Kansan. From the fact, however, that it
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HISTORY OF DECATUR COUNTY
shows a certain amount of rude stratification, as well as the fact that the soil horizon is about on a level with the present flood plain, the exposure may perhaps be thought to represent side filling in over the bottom land. The absence of direct evidence favoring this, and the fact that so large an amount of bowlder clay could hardly slip down without leaving direct evidence of the fact except by a remarkably slow and uniform movement, while the bottom land is evidently young, seems sufficient reason for rejecting this hypothesis.
There is another exposure of interest found in the east bluff of the river near the bridge, about four miles southeast of Davis City (southwest of northwest section 18, Hamilton Township). Above the bridge there is a small ravine coming in from the east and cutting in two what was once apparently a continuous exposure. The portion of this exposure south of the ravine shows at the base a blue black bowlder clay with many pebbles. This clay has the typical character- istics of the pre-Kansan. Its blackness here is quite noticeable and leads one on first view to expect a Carboniferous shale. It does not extend along the entire base of the exposure and seems to be sep- arated from the remainder of the latter by a zone of weathering. Over it where first seen are beds of stratified sand, gravel, and loess with at least one pretty well marked zone of weathering. North of the ravine is a blue bowlder clay, not so dark in color, breaking cubically rather than in flakes, and passing upward into a yellow bowlder clay containing masses of highly weathered gravels of Aftonian aspect. Then yellowing, resultant on oxidation, here fol- lows the cracks well down into the blue elay. In the adjoining region the usual succession of loess, gumbo, yellow and blue bowlder clay is seen. The compact black flaky bowlder elay is unusual. At the exposure itself the facts are not altogether clear, but this much may be stated definitely, that there is here a bowlder clay of a type uncom- mon for this region but of physical character very like that of the older drift at Afton Junction.
Directly west of Leon, on the main road to Decatur City (south west southwest section 29, Center Township), a long westward facing slope shows the exposure sketched below.
On the top of the hill is the usual upland loess (1) running down over the edge of the rather steep slope. Below it is the normal gumbo deposit (2) eight to ten feet thick. Under this is a yellow bowlder clay (3) with all the usual characteristics. So far the section is ex- actly the same as oecurs throughout the county. The bowlder clay is, however, only about fifteen feet thick, and below it is found a
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HISTORY OF DECATUR COUNTY
second gumbo about twelve feet thick. This is a dark blue- drab clay. At its upper limit it contains humus and a distinct soil three to five inches thick. The soil is quite black and well marked, though thin. It contains some roots which do not seem to come down from the bowlder clay. The latter shows slight evidence of water action for as much as a foot above the gumbo, but above that is the normal unstratified bowlder clay. Under the gumbo is a second yellow bowlder clay (6) not differing in any known particular from that above. It carries cherts, red and gray granites, limestones, greenstones, iron concretions and quartzites. The same sorts of rocks are found in the clay above. Both show evidence of age and carry much weathered material. At the foot of the slope is the alluvium of the bottomland.
The ravines at the side of the road have cut back far enough to show that the beds lie directly under each other as indicated. The upper bowlder clay (3) where it rests upon the lower gumbo (4) is not the hillside wash or the result of creep. The material brought down by these processes is shown at 5 and is quite distinct. It includes smaller pebbles, is sandier, very gravelly, and distinctly waterlaid. It can be distinguished at a glance. No hypothesis of slipping seems able to account for the arrangement of the beds and they seem to indi- cate true and original superposition. This is the more probable from the fact that exactly similar exposures, except that the relations are even clearer, may be seen about one and a half miles east of Osceola in Clarke County. At several points in the ravines north of Weldon the same phenomena seem to be present though the exposures are not good. Only at the Leon exposure was the soil on top the lower gumbo noted. It has here the appearance of a buried soil with the upper portion removed, leaving only a little of the soil proper over the subsoil. There is no sufficient evidence of erosion at any point in the section lower than the top of the upper bowlder clay.
In regions where the superimposed drift sheets occur, buried forests are not uncommonly encountered. This is particularly true in regions near the edge of an upper drift, where, probably as a result of the fact that but little ice passed over the forest, it is better preserved. Buried forests are not of equal significance. They may readily occur as a result of temporary retreats and advances of the ice where only one drift sheet is present. It is only when they throw light upon the climate or physical conditions obtaining during the interrum that they have important bearing. It should always be remembered, however, that the simplest explanation is not neces-
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HISTORY OF DECATUR COUNTY
sarily the true one, and that where the facts are capable of explana- tion equally well by the hypothesis of one or of two ice sheets, it is by no means necessarily true that the former hypothesis is to be preferred.
There are evidences of a buried forest in Decatur County, and in the adjoining region. Indeed, such evidence is found at a number of points in Southern Iowa, and has been reviewed at another place."" In Decatur County the forest bed is best known in the vicinity of Lamoni, where it has been encountered in several wells. In the eleva- tor well at that place it was struck at a depth of eighty-five feet, and below it there was a thickness of 100 feet of bowlder elay. It is clear that this forest bed is far below the base of the loess and is in the bowlder clay. There are no specimens of wood at hand, though the material examined by Prof. T. J. Fitzpatrick was found to be conif- erous. The climatic bearing of the find is unimportant. The signifi- cant facts are that the bed is of some thickness, occurs commonly in the deep wells over quite a wide region, and is in the bowlder elay. It evidently neither represents adventitious wood in the latter, nor any pose-Kansan accumulations.
In Harrison County, Mo., Dr. C. R. Keyes 33 reports a nine-foot forest bed struck at a depth of about one hundred and twenty feet and in the drift. The evidence here would seem to be of the same nature as at Lamoni, but the thickness of the bed makes more impos- sible any reference of the deposits to adventitious sources, and indi- cates some little time of accumulation.
These two cases represent the better examples of buried forests in Decatur and its immediate vicinity. Other cases are reported, but do not seem so reliable. In Union County good specimens of peat have been obtained from wells near Afton, but the horizon is not well fixed and may be of later age. Setting aside for the present the buried gumbo near Leon, it will be noted that there are in this county or its immediate vicinity the following evidences of two drifts.
1. Waterlaid deposits between tills.
2. Buried forests and soil horizons.
3. Traces of an underlying till of peculiar and marked physical character.
In considering the first of these it will at once suggest itself that the large amount of ice necessitates considerable water-action (though not necessarily "great floods"), and that accordingly waterlaid beds
32 Proc. Iowa Acad. Sci., Vol. V.
33 Private communication.
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HISTORY OF DECATUR COUNTY
may be expected to occur at various horizons in and about the drift. It is possible, however, that the deposits should be of such a nature as clearly interdict any reference to ice-derived floods in their forma- tion, or their distribution might be such as to show that they followed a considerable period of erosion. Neither is exactly true in this case, but it is true that the gravels found above Westerville are of the same character and occupy the same position as those found at Afton, and there are some reasons for believing that the latter accumulated dur- ing a considerable period of erosion.
Regarding the evidence derived from buried soils and forest beds but little can be added to what has already been said. It is mani- festly uncertain and of slight independent value.
The third point is one hard to estimate. It is true, however, that whatever one may think of correlations based upon the color and physical characteristics of bowlder clays, there is certainly some sig- nificance in the fact that at every known exposure in Iowa, of bowlder clays which for various reasons are considered as probably older than the Kansan, the physical character of the bowlder clay is the same, and that it is markedly different from that of the Kansan.
This is true not only of such clays in southwestern Iowa but of the exposures at Albion in Marshall County, Oelwein in Fayette, and at Muscatine. It is certainly a fact of some significance. Probably none of these classes of evidence at this point would independently prove the presence of a pre-Kansan drift, but it must be remembered the facts have a cumulative value. If, for example, a single exposure showed a forest bed, a soil and waterlaid deposits between drift sheets of markedly different physical characters, and there were no opposing phenomena in the surrounding region, but one inference could be drawn. In the same way when the three classes of phenomena occur not in the same, but in contiguous exposures, they gather weight from the association. For this reason it is believed that the evidence from Decatur County, meager though it admittedly is, supports the hypothesis that there are traces of a pre-Kansan drift sheet in the region, separated from the Kansan by an unknown but probably important interval.
The exposures near Leon, it is believed, are best interpreted as results of changes in the front of the Kansan ice sheet. The gumbo alone proves only that there was a period when fine sedimentation such as is characteristic of still waters could go on for some time. The soil has been so nearly removed that its original thickness can only be guessed, and it is recognized that soils alone do not necessarily
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HISTORY OF DECATUR COUNTY
indicate an especially long lapse of time. The thickness of the overlying till and the total lack of distinguishing marks between it and that below the gumbo throws the exposure out of harmony with those of the Aftonian and pre-Kansan beds. The apparently local nature of the phenomena, confined as they are to a relative narrow belt stretching from Osceola to Leon, suggests a local cause.
In the recent railway cuts of the D. M. & K. C. Railway there are, at a few points, gravels suggestive of the Buchanan. The gravel consists of small well rounded pebbles, is highly stained, carries Feathered material, and occurs apparently in pockets in the top of the Kansan and under the gumbo. It has the appearance at times of local hillside wash; but its occurrence at such widely scattered joints as Leon, New Virginia, and Truro, together with the fact that in Eastern Iowa the Buchanan gravels often occur some miles out from the edge of the later Iowan drift, suggests the advisability of keeping in mind the alternative hypothesis.
Glacial striæ .- The limestone on Pot Hole Creek at one point shows stria as indicated in Plate xxiii. As measured by Prof. T. J. Fitzpatrick these have a direction of s. 1º w. magnetic. They are upon the Winterset limestone and below the Kansan drift.
LOESS AND GUMBO
The only general deposits occurring throughout the county and later than the drift are the loess and the gumbo clay. They are of the general type familiar throughout Southern Iowa and Northern Missouri. The loess is of the older or white clay phase, and as com- pared with that found along the Mississippi and Missouri rivers as well as inland farther north, is less porous, more plastic, and non- fossiliferous. It carries lime nodules but is free from pebbles. It graduates upward into the black loam which forms the prairie soil.
The gumbo belongs stratigraphically with the loess. It occurs below the latter, and has a blue to drab color. It is even more plastic and less porous than the loess. When damp but not wet. it has a mealy appearance which is quite deceptive as to its real character. It rarely carries pebbles though a few have been found in it. It often contains small lime balls but these are neither so large nor so numer- ous as in the loess. It has the appearance of being finer grained than the latter and suggests a quiet water deposit which has since been compacted or puddled by water. In general the gumbo is about ten feet thick and rests on the ferretto horizon of the Kansan. The Vol. I .- 17
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HISTORY OF DECATUR COUNTY
loess is from ten to as much as twenty feet thick. Both deposits passed down the flanks of the hills into the larger valleys.
ALLUVIUM
The alluvial deposits of Decatur County while extensive have little that is peculiar. They cover the broad bottoms of Grand, Wel- don and Little rivers, and occur along many of the minor streams. As a rule the alluvium is not of any remarkable thickness. Along Grand River the flood plain is usually about fifteen feet above ordinary water stages. The alluvium is necessarily made up in the main of material derived from the loess and gumbo. South of Davis City, however, along Dickerson Creek, it contains large bodies of sand and gravel, derived apparently from beds of the same age as the gravels above Westerville. Inasmuch as the river does not show this material in the region between Westerville and Davis City, it is highly probable that the beds which formed the source of the Dickerson Creek deposits are concealed below the drift in the hills west of Davis City.
STRUCTURE
The rocks of the county have been subject to very little disturb- ance. The dip noted west of Decatur (see Plate xxiv) is the most pronounced in the county. It is entirely local and throughout the area the rocks lie very nearly horizontal. Apparently the general dip to the southwest which characterizes the rocks of so much of the state is here almost entirely absent. There are no data which war- rant considering it here to be more than one or two feet per mile. The base of the Bethany, so far as Decatur County is concerned, seems to occupy a practically horizontal plane.
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