Mills County, Iowa

1881 Mills County History
Geology

  The geological history of Mills county is one of peculiar interest, and affords some very suggestive facts relative to its past vicissitudes. It extends in point of time over many thousands of years, and embraces periods of repose and periods of remarkable change. Its history, climatologically, has been one of deep interest, and embraces changes so radical and so directly at variance with one another as to be almost incredible.

There have been long ages when it basked under a torrid sun, and then these ages gave place to others equally as remarkable for polar frosts. Life, in all the variety and luxuriance of a tropical climate, gave place to the desert wastes of an arctic zone. Nor were these changes sudden. They are there, stamped in the very rocks at your door, and limned upon the landscape of your valleys, not as great and far-reaching catastrophes, but as gradual transitions, indisputably marked as such by the fossil forms that roll out from the rock you crush, or see traced with a delicacy no draughtsman can imitate. There have been times when Old Ocean, heedless of his doings, dashed against the rocky barrier that dared dispute his sway, or rolled in solemn, conscious might above its highest point; times when a beautiful and varied flora thrived on its surface; and times when there was naught save a waste of desert matter. We strike our pick in the shales on the hillside, and behold! There in the coal that gives us warmth and drives our engines, are the fairy forms that made the fern paradise of the coal period – beautiful arguments, those, of changes that thousands of years, as we measure time, would not compass. In presenting the following principal facts in the geology of Mills county, enough only of the lithological characters of the various rock strata have been given to enable the interested reader to identify them. Many points of interest from a geological standpoint have necessarily been omitted; their introduction would have unduly lengthened the chapter, and scarcely possessed any general interest. To trace, briefly, the changes that have occurred, and to note their probable causes, are the main purposes of this sketch.

The surface of the county is entirely covered with the deposit before referred to as the “bluff deposit.” But perhaps more correctly the loess. It lies next above the drift and varies in depth, in different parts of the county, from five to one hundred feet. In appearance the deposit is peculiarly characteristic, presenting substantially the same features in whatsoever part of the globe it is found. Its material is of a slightly yellowish ash color, except where darkened by decaying vegetation, very fine and silicious, but not sandy, “not very cohesive and not at all plastic.” Along the Missouri bottom the formation is exposed in the most favorable manner for study. Those bold, high escarpments stand out as monuments – not ver endurable, to be sure – to mark the great changes that have occurred in the surface features of this county. Mixed throughout this material are to be found various species of land and fresh water shells that seem to furnish a clue to a solution of the problem concerning its origin. Relative to this point, it is sufficient for present purposes to simply indicate the more prominent points in the theories broached, of which there are two principal ones. The first, and, to speak within bounds, a most novel one, is the theory of Baron von Richthofen. The Baron’s theory, based principally upon the study of the loess of China, is substantially this: “that loess, certainly in China, and probably in all continents, is a sub-aerial deposit collected on dry grassy areas by the action of fierce winds. For the formation of such a region he supposes a central undrained elevated area, from which nearly all moisture is excluded by surrounding mountain chains.”

To this theory is opposed what is called the sub-aqueous theory, which the reader will at once notice is diametrically opposite to that of the Baron. Without entering into the details of the various arguments advanced by those who maintain the last named theory, it is sufficient to say that the lacustrine origin of the loess is now quite a generally conceded point. Such an origin involves radical changes in our conceptions of the physical aspect of the county. We must conceive the present level of the land to be somewhat lowered, the waters of the Missouri barred on their way to the ocean, spreading eastward and westward until they assumed the proportions of a great inland sea, two hundred or more miles in length. Far away to the northwest the upper Missouri is plowing its way through the land, wearing away its boundaries and hurrying onward with them to the comparatively quiet waters below. The depression of the land meant also the northward extension of the Gulf of Mexico, which, then as now, became the final recipient of the waters of the Missouri. In the great Lake Missouri the finely comminuted material held in suspension by its waters was deposited as a blanket of silt over the bottom of the lake – the former surface of the land. Then came those giant throes which lifted again the partially submerged continent, hurled the encroaching waves of the ocean back to their former dominion, and allowed the waters of the ancient Lake Missouri to gradually reach the sea. Then began a period of erosion, not yet ended, by means of which the great river has plowed out its present valley through the land. The abrading process still continues on a scale to enormous as to excite our wonder, and it is the immediate cause that renders so treacherous and uncertain the navigation of the stream. Through sediment of its own deposition in centuries far back in the history of time the river is cutting its way, changing its channel ever and anon, and carrying in its turbid waters much of the land of Mills to make fertile the broad acres along its lower course.

The lake the river formed in the far off past was not a lake of an hour, nor one of a season of floods, but for centuries reigned where now the farmer guides his plow. It contained life-forms, many of which, or closely allied ones, are living to-day. Among them flourished shells of the genera Physa, Limnophysa, Planorbis, and perhaps Ancylus. These are found throughout the Loess mingled with land shells of the general Mesodon, Succinea, Zonites and others. Prof. J. E. Todd, in the Proceedings of the American Association for the Advancement of Science, Vol. XXVII, reports twenty-seven species from the Loess of Fremont county Prof. Samuel Aughey reports a list of one hundred and twenty-three, of which seventy-eight at least are incorrectly determined. Not more than forty-five of all the forms he has listed in his “Sketches of the Physical Geography and Geology of Nebraska,” p. 287, can possibly stand. Is it questioned how came these land shells here? They were brought down by floods from the higher and wooded sections forming the boundaries of the lake, and at length sinking to the bottom were covered with silt in manner similar to that which entombed their allied brethren of the fresh water forms. These remains are in themselves almost conclusive proof of the fresh water origin of the Loess, and help to solve some of the questions of the surface geology of Mills.

Immediately beneath the Loess is found the Drift, though rarely seen in Mills county, and then only in deep railroad cuts or in the valleys or those streams which have eroded their courses to a great depth. The terms “drift,” as it is commonly employed in geology, “includes the sand, gravel, clay and boulders occurring over same parts of the continents, which are without stratification or order of arrangement, and have been transported from places in high latitudes by some agency which (1) could carry masses of rock hundreds of tons in weight, and which (2) was not always dependent for motion on the slopes of the surface.” (Hall.) This agency was ice, either in the form of an extensive glacier or detached masses called icebergs. The whole surface of North America, to the thirty-ninth parallel, bears evidence of the denuding and transforming power of this agency. This it was which rounded, in part, these hills, partially filled old valleys or dug out new ones, and which left at our very doors these masses of rock – large and small – or buried them in the hillside, to excite our wonder and cause us to speculate as to their origin. They were brought hither from some northern locality where the material from which they were derived may be found in situ. Often there are found, in the valleys of the deeper streams, and in “land-slides” along the “bluffs” on the Missouri bottom large masses of Sioux Quartzite, and rocks of other kinds, from points still farther to the northward. The general direction of the glacial movement was southward. In section 16, tp. 71, r. 43 west, are found “two distinct sets of scratches upon the same surface and crossing each other, “ showing that the movement of the glacier changed while passing over this rock – which is one of the series of the upper coal measure limestone. The “one set has a direction south, twenty degrees east, and the other, south, fifty-one degrees east.”

The exposures of the drift in the county are quite inconsiderable and always local. It is nowhere the surface soil, and is to be seen only in the deepest valleys or at the base of the loess along the bluffs. It is occasionally seen along the course of the Nishnabotna, and frequently exposed in the numerous ravines in the vicinity of Glenwood, and indeed wherever there are deeply eroded valleys among the bluffs. Where it appears it is seen to be a compound of clay and gravel, with occasional beds of sand, and is deposited without regularity – being what is technically termed unmodified drift. It usually contains many small and well-worn pieces of gneiss, porphyry, hornblende, and other primary rocks, together with occasional small fragments of limestone, sandstone, and bits of slate, all of which are of much older ages, and have been transported from points more or less remote from their present locality. The bluffs along the Mississippi are almost entirely composed of the drift, a most striking difference between them and those along the Missouri, which are, superficially at least, composed of the loess.

There being no rocks of Permian, Triassic, or Jurassic age in this county, or indeed in the state, the series next met with belong to another period of geological time, older far than any yet considered – the lower Cretaceous. They are the Nishnabotna sandstones and are named for the river along whose course they appear, and where they have been studied. Lithologically, the formation is a course-grained friable, and ferruginous sandstone. The presence of a very large amount of oxide of iron gives the rock a sombre and displeasing color which, were it fitted in other respects, would greatly lessen its value for building purposes. In the northeastern part of this county it has, however, been quite extensively quarried, and being of somewhat better quality than the same formation in other sections, makes a fair building stone. It has little economic value. There is, nevertheless, a fact that should not be overlooked. It lies unconformably upon the rocks of the upper coal-measures, and does not partake of the dips of the older formations, but has one of its own – to the north of westward. The formation, being at the surface during the glacial epoch, suffered a most extensive denudation, but this is the very feature that has added value to it, for the sand thus derived has contributed greatly to the mellowness and warmth of the soil, and largely increased its productiveness.

Of the Coal measures, which lie next below the Cretaceous in Iowa, only the Upper Coal measure strata have been exposed in this county, and as would be naturally inferred, the thickness of the superincumbent loess and drift negatives the probability of either numerous or extensive outcrops. There is so little difference in the geological and physical features of this and Fremont counties that the following account of the Coal measures of the latter will be of exact application here. There is, in addition, the fact that no section of the measures indicated has ever been made in this county, so complete as that which here follows:

“None [of the coal-measure strata] have been found in the valleys of either of the Nishnabotnas, and, with the exception of a slight one in the valley of Walnut creek, the only exposures are to be found at distant intervals along the base of the bluffs that border the Missouri river flood-plain. They usually extend only a few feet in height above the level of the plain, and are then lost from sight beneath the bluff deposit, or the slight intervening accumulation of drift; but in the northwestern part of the county a few exposures reach considerable height above the general level of the flood-plain.

On the land of John Wilson, section 23, township 70, range 43, there are some fine exposures of upper coal-measure strata, which reach the greatest aggregate thickness of any yet known within the state, westward from Madison county. It is, therefore, a locality of great interest and importance in the study of that formation in southwestern Iowa. The strata observed there are represented by the following:

• SECTION NEAR WILSON’S
• No. 29 – Yellowish grey, impure limestone, in thin layers . . . . . . . . . . . . . . . . . . . . . . . . . 2 feet
• No. 38 – Limestone in two layers, with a three-inch marly parting . . . . . . . . . . . . . . . . . . 2 ½ “
• No. 27 – Yellowish shaly marl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... 1 ¼ “
• No. 26 – Black carbonaceous shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...1 ¾ ‘
• No. 25 – Bluish clayey shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......1 ¼ “
• No. 24 – Black carbonaceous shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...1 “
• No. 23 – Bluish, marly shale, with numerous fossils . . . . . . . . . . . . . . . . . . . . . . . . . . . ......1 ½ “
• No. 22 – Impure coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......10-12 “
• No. 21 – Light bluish, fossiliferous, shaly clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......2 “
• No. 20 – Compact, bluish limestone with shaly partings . . . . . . . . . . . . . . . . . . . . . . . . . ....4 “
• No. 19 – Marly clay, with calcareous concretions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......6 “
• No. 18 – Light gray limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........4 “
• No. 17 – Unexposed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........6 “
• No. 16 – Compact limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........1 ½ ”
• No. 15 – Light yellowish indurated marl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........4 “
• No. 14 – Yellowish silicious limestone with flinty concretions . . . . . . . . . . . . . . . . . . . . .........2 ½ “
• No. 13 – Yellowish, marly shale, with concretions of impure limestone . . . . . . . . . . . . . .......3 “
• No. 12 – Compact limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........1 “
• No. 11 – Yellowish marly shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........2 “
• No. 10 – Gray limestone in thick layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......3 “
• No. 9 – Bluish clayey shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........1 ½ “
• No. 8 – Yellowish silicious limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........¾ “
• No. 7 – Compact gray limestone, with marly partings . . . . . . . . . . . . . . . . . . . . . . . . . . . .........16 “
• No. 6 – Bluish, shaly clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............1 ½ “
• No. 5 – Compact layer of limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............¾ “
• No. 4 – Bluish, shaly clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............2 ½ “
• No. 3 – Compact, bluish limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............2 “
• No. 2 – Bluish clayey shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............4 “
• No. 1 – Fine grained, micaccous sandstone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........1 “
• Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............80 10-12 ft

The lowest member of the foregoing section, No. 1, is only a few feet above the general level of the flood-plain, and many other members appear successively above it in the fact of the bluff that fronts the flood-plain near Mr. Wilson’s residence, while the higher members are found in the bed and banks of a small rivulet that comes down from the uplands through the bluffs at this point. Along the base of the bluffs above this point, as far as the north boundary line of the county, frequent exposures are seen of strata which are equivalent to a large part of those that constitute the lower half of the section at Wilson’s. Southward from Wilson’s, along the base of the bluffs, several exposures of the lowest members of that section are seen at long intervals, always holding about the same relative position above the level of the flood-plain, all the way to the town of Hamburg, in the southwestern part of the county. The fine-grained micaseous sandstone of No. 1, of the foregoing section, is seen at the village of Plum Hollow, a couple of miles below Mr. Wilson’s, and also at Hamburg, and several intermediate points. A few strata were found resting upon it at all these points, but none were observed beneath it. It is therefore stratigraphically the lowest stratum found in southwestern Iowa, if we except the lowest strata of Madison and Decatur counties. It is regarded as equivalent with No. 1 of the section in the valley of the Tarkio, in Page county; with No. 2 of the section at Winterset, in Madison county, and with No.2 of the section at Davis’ Mills, in Decatur county. It will thus be seen that lowest stratum exposed in Fremont county is regarded as equivalent with strata that further eastward are known to be near the base of the Upper coal-measures. It is, of course, inferred that that stratum is also near the base of the same formation, unless those beneath it have thickened very greatly in their westward extension.

No doubt is entertained that the thin bed of impure coal represented by No. 22, of the section at Wilson’s, is identical with the bed of coal that has been opened at various points along the valley of the Nodaway, from the center of Adams county to the southern boundary of the state. The horizon of this coal is referred to near the base of the serious of limestone strata exposed near Winterset, in Madison county. If this reference is correct, it will be seen that there is a greater aggregate thickness of limestone strata in Fremont county, beneath the horizon named, than there is in Madison county, which seems plainly to indicate a thickening of the strata of the upper coal measures to the westward.”

There is, therefore, no probability that workable beds of coal will be found in this county, at a point near enough to the surface to make mining profitable. In the deeper ravines and gullies the strata may be reached by deep mining, but at many hundreds of feet. In borings made some years ago in the vicinity of Nebraska City, the drill is said to have penetrated four hundred feet, and then only reach the middle coal-measure which must first be pierced before the coal-bearing strata are reached in this State, the base of the lower coal-measures. Wood must continue to be the main reliance for fuel until transportation rates have been so reduced as to make the introduction of coal from other portions of the state economical.

Hence among the mineral resources of this county, coal cannot be included, but is effectually settled in the negative by the reasons and facts above adduced. The only available material of an economical nature, aside from the fertile soil, are the limestone out crops along the Missouri bottom, in southwest part of the county, and a few minor exposures of fair building material, above referred to as the Nishnabotna sandstone in the extreme northeast and along Farm Creek. The fertility and richness of the soil is absolutely inexhaustible from an agricultural point of view for many feet down, or, in other words, throughout the entire loess formation. That it, together with the occasional good deposits of clay which are found at the base of the bluffs, may be successfully used in the manufacture of an excellent quality of brick, is sufficiently attested to by the numerous fine dwellings and business houses in the county, constructed of native brick. It will always be true of Mills county that its wealth is in its broad acres, as being so well adapted to agriculture, rather than in any hidden sources of mineral wealth. The crack of the whip rather than the hum of wheels, the sturdy arm of the farmer rather than the pick of the miner must be its almost sole reliance.

Thus briefly has been given all that is definitely known of the geology of this county. It presents many features of interest, and promises to abundantly reward any person willing to complete a minute survey of its domain.

A word as to the forms of life found imbedded in the rocks of the county. The fossils characteristic of the Upper Coal-measure strata may be obtained at every point – where the rocks are exposes, and these exposures promise a rich harvest to the student of ancient life, as well as to the mere curiosity hunter. Here are found the now extinct and remarkable trilobites (Phillipsia) – a genus of fossil crustaceans allied to the modern “horse-shoe crab” of the Atlantic coast; and a very beautiful and curious form of coral (Campophyllum torquium), a silent witness to a once tropical climate. Among the articulates are found several species of Productus, once classed – and still by many – with the mollusca, but now beginning to be recognized as closely allied to the worms. Others of the brachiopods are Chonets granulifera and C. glabra. Among the “flowers of the carboniferous world” were crinoids – stemmed echinoderms – of which the living Pentacrinus asteria of the West Indian seas is a type; not flowers at all, though popularly called “stone lilies,” but an animal. These all point to a time when the ocean covered the county and the rocks in which they are found were being formed. They are full of instruction for those who will carefully study them.

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