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A part of the IAGenWeb and USGenWeb Projects The State of Iowa Geology |
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A part of the IAGenWeb and USGenWeb Projects The State of Iowa Geology |
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The soil of Iowa may be separated into three general divisions, which not only possess different physical characters, but also differ in the mode of their origin. These are drift, bluff and alluvial, and belong respectively to the deposits bearing the same names. The drift occupies a much larger part of the surface of the State than both the others. The bluff has the next greatest area of surface, and the alluvial least.
All soil is disintegrated rock. The drift deposit of Iowa was derived, to a considerable extent, from the rocks of Minnesota; but the greater part of Iowa drift was derived from its own rocks, much of which has been transported but a short distance. In general terms the constant component element of the drift soil is that portion which was transported from the north, while the inconstant elements are those portions which were derived from the adjacent or underlying strata. For example, in Western Iowa, wherever that cretaceous formation known as the Nishnabotany sandstone exists, the soil contains more sand than elsewhere. The same may be said of the soil of some parts of the State occupied by the lower coal measures, the sandstones and sandy shales of that formation furnishing the sand.
In Northern and Northwestern Iowa, the drift contains more sand and gravel than elsewhere. This sand and gravel was, doubtless, derived from the cretaceous rocks that now do, or formerly did, exist there, and also in part from the conglomerate and pudding-stone beds of the Sioux quartzite.
In Southern Iowa, the soil is frequently stiff and clayey. This preponderating clay is doubtless derived from the clayey and shaly beds which alternate with limestones of that region.
The bluff soil is that which rests upon, and constitutes a part of, the bluff deposit. It is found only in the western part of the State, and adjacent to the Missouri River. Although it contains less than one per cent. of clay in its composition, it is in no respect inferior to the best drift soil.
The alluvial soil is that of the flood plains of the river valleys, or bottom lands. That which is periodically flooded by the rivers is of little value for agricultural purposes; but a large part of it is entirely above the reach of the highest floods, and is very productive.
The stratified rocks of Iowa range from the Azoic to the Mesozoic, inlcusive; but the greater portion of the surface of the State is occupied by those of the Palĉozoic age. The table below will show each of these formations in their order:
Ages. |
Periods. |
Epochs. |
In Feet. |
| Cretaceous......... " " " " " " " Upper Silurian..... Lower Silurian..... " " " " |
Post Tertiary......... Lower Cretaceous. " " " " Niagara.............. Cincinnati............ " Primordial. " " |
Drift................................. Inoceramous bed.................... Woodbury Sandstone and Shales... Nishnabotany Sandstone............ Upper Coal Measures............... Middle Coal Measures............... Lower Coal Measures............... St. Louis Limestone................. Keokuk Limestone................... Burlington Limestone................ Kinderhook beds..................... Hamilton Limestone and Shales..... Niagara Limestone.................. Maquoketa Shales................... Galena Limestone.................... Trenton Limestone.................. St. Peter's Sandstone............. Lower Magnesian Limestone......... Potsdam Sandstone.................. Sioux Quartzite..................... |
50 130 100 200 200 200 75 90 196 175 200 350 80 250 200 80 250 300 50 |
The Sioux quartzite is found exposed in natural ledges only upon a few acres in the extreme northwest corner of the State, upon the banks of the Big Sioux River, for which reason the specific name of Sioux Quartzite has been given them. It is an intensely hard rock, breaks in splintery fracture, and a color varying, in different localities, from a light to deep red. The process of metamorphism has been so complete throughout the whole formation that the rock is almost everywhere of uniform texture. The deip is four or five degrees to the northward, and the trend of the outcrop is eastward and westward. This rock may be quarried in a few rare cases, but usually it cannot be secured in dry forms except that into which it naturally cracks, and the tendency is to angular pieces. It is absolutely indestructible.
Potsdam Sandstone.--This formation is exposed only in a small portion of the northeastern portion of the State. It is only to be seen in the bases of the bluffs and steep valley sides which border the river there. It may be seen underlying the lower magnesian limestone, St. Peters sandstone and Trenton limestone, in their regular order, along the bluffs of the Mississippi from the northern boundary of the State as far south as Guttenburg, along the Upper Iowa for a distance of about twenty miles from its mouth, and along a few of the streams which empty into the Mississippi in Allamakee County.
It is nearly valueless for economic purposes.
No fossils have been discovered in this formation in Iowa.
Lower Magnesium Limestone.--This formation has but little greater geographical extent in Iowa than the Postdam sandstone. It lacks a uniformity of texture and stratification, owing to which it is not generaly valuable for building puroses.
The only fossils found in this formation in Iowa are a few traces of crinoids, near McGregor.
St. Peter's Sandstone.--This formation is remarkably uniform in thickness throughout its known geographical extent; and it is evident it occupies a large portion of the northern half of Allamakee County, immediately beneath the drift.
Trenton Limestone.--With the exception of this, all the limestones of both Upper and Lower Silurial age in Iowa are magnesian limestones -- nearly pure dolomites. This formation occupies large portions of Winnesheil and Allamakee Counties and a portion of Clayton. The greater part of it is useless for economic purposes, yet there are in some places compact and evenly bedded layers, which afford fine material for window caps and sills.
In this formation, fossils are abundant, so much so that, in some places, the rock is made up of a mass of shells, corals and fragments of trilobites, cemented by calcareous material into a solid rock. Some of these fossils are new to science and peculiar to Iowa.
The Galena Limestone.--This is the upper formation of the Trenton group. It seldom exceeds twelve miles in width, although it is fully one hundred and fifty miles long. The outcrop traverses portions of Howard, Winnesheil, Allamakee, Fayette, Clayton, Dubuque and Jackson. It exhibits its greatest development in Dubuque County. It is nearly pure dolomite, with a slight admixture of silicious matter. It is usually unfit for dressing, though sometimes near the top of the bed good blocks for dressing are found. This formation is the source of the lead ore of the Dubuque lead mines. The lead region proper is confined to an area of about fifteen miles square in the vicinity of Dubuque. The ore occurs in vertical fissures, which traverse the rock at regular intervals from east to west; some is found in those which have a north and south direction. The ore is mostly that known as Galena, or sulphuret of lead, very small quantities only of the carbonate being found with it.
Maquoketa Shales.--The surface occupied by this formation is singularly long and narrow, seldom reaching more than a mile or two in width, but more than a hundred miles in length. Its most southerly exposure is in the bluffs of the Mississippi near Bellevue, in Jackson County, and the most northerly yet recognized is in the western part of Winnesheil County. The whole formation is largely composed of bluish and brownish shales, sometimes slightoy arenaceous, sometimes calcareous, which weather into a tenacious clay upon the surface, and the soil derived from it is usually stiff and clayey. Its economic value is very slight.
Several species of fossils which characterize the Cincinnati group are found in the Maquoketa shales; but they contain a larger number that have been found anywhere else than in these shales in Iowa, and their distince faunal characteristics seem to warrant the separation of the Maquoketa shales as a distant formation from any others of the group.
Niagara Limestone.-The area occupied by the Niagara limestone is nearly one hundred and sixty miles long from north to south, and forty and fifty miles wide.
This formation is entirely a magnesian limestone, with in some places a consideral proportion of silicious matter in the form of chert or coarse flint. A large part of it is evenly bedded, and probably affords the best and greatest amount of quarry rock in the State. The quarries at Anamosa, LeClaire and Farley are all opened in this formation.
Hamilton Limestone.--The area of surface occupied by the Hamilton limestone and shales is fully as great as those by all of the formation of both Upper and Lower Silurian age in the State. It is nearly two hundred miles long and from forty to fifty miles broad. The general trend is northwestward and southeastward.
Although a large part of the material of this formation is practically quite worthless, yet other portions are valuable for economic purposes; and having a large geographical extent in the State, is one of the most important formations, in a practical point of view. At Waverly, Bremer County, its value for the production of hydraulic lime has been practically demonstrated. The heavier and more uniform magnesian beds furnish material for bridge piers and other material requiring strength and durability.
All the Devonian strata of Iowa evidently belong to a single epoch, and referable to the Hamilton, as recognized by New York geologists.
The most conspicuous and characteristic fossils of this formation are brachiopod, mollusks and corals. The coral Acervularia Davidsoni occurs near Iowa City, and is known as "Iowa City Marble," and "bird's-eye marble."
Of the three groups of formations that constitute the carboniferous system, viz., the subcarboniferous, coal measures and permian, only the first two are found in Iowa.
The area of the surface occupied by this group is very large. Its eastern border passes from the northeastern part of Winnebago County, with considerable directness in a southeasterly direction to the northern part of Washington County. Here it makes a broad and direct bend nearly eastward, striking the Mississippi River at Muscatine. The southern and western boundary is to a considerable extent the same as that which separates it from the coal field. From the southern part of Pocahontas County it passes southeast to Fort Dodge, thence to Webster City, thence to a point three or four miles northeast of Eldora, in Hardin County, thence southward to the middle of the north line of Jasper County, thence southeastward to Sigourney, in Keokuk County, thence to the northeastern corner of Jefferson County, thence sweeping a few miles eastward to the southeast corner of Van Buren County. Its area is nearly two hundred and fifty miles long, and from twenty to fifty miles wide.
The Kinderhook Beds.--The most southerly exposure of these beds is near the mouth of the Skunk River, in Des Moines County. The most northerly now known is in the eastern part of Pocahontas County, more than two hundred miles distant. The principal exposures of this formation are long the bluffs which border the Mississippi and Skunk Rivers, where they form the eastern and northern boundary of Des Moines County, along English River, in Washington County; along the Iowa River, in Tama, Marshall, Hamlin (sic - Hardin) and Franklin Counties; and along the Des Moines River, in Humboldt County.
The economic value of this formation is very considerable, particularly in the northern portion of the region it occupies. In Pocahontas and Humboldt Counties it is almost invaluable, as no other stone except a few boulders are found here. At Iowa Falls the lower division is very good for building purposes. In Marshall County all the limestone to be obtained comes from this formation, and the quarries near LeGrand are very valuable. At this point some of the layers are finely veined with peroxide of iron, and are wrought into ornamental and useful objects.
In Tama County, the oolitic member is well exposed, where it is manufactured into lime. It is not valuable for building, as upon exposure to atmosphere and frost, it crumbles to pieces.
The remains of fishes are the only fossils yet discovered in this formation that can be referred to the sub-kingdom VERTEBRATA; and so far as yet recognized, they all belong to the order selachians.
Of ARTICULATES, only two species have been recognized, both of which belong to the genus phillipsia.
The sub-kingdom MOLLUSCA is largely represented.
The RADIATA are represented by a few crinoids, usually found in a very imperfect condition. The sub-kingdom is also represented by corals.
The prominent feature in the life of this epoch was molluscan; so much so in fact as to overshadow all other branches of the animal kingdom. The prevailing classes are: lamellibranchiates, in the more arenaceous portions; and brachiopods, in the more calcareous portions.
No remains of vegetation have been detected in any of the strata of this formation.
The Burlington Limestone.--This formation consists of two distinct calcareous divisions, which are separated by a series of silicious beds. Both divisions are eminently crinoidal.
The southerly dip of the Iowa rocks carries the Burlington limestone down, so that it is seen for the last time in this State in the valley of Skunk River, near the southern boundary of Des Moines County. The most northerly point at which it has been recognized is in the northern part of Washington County. It probably exists as far north as Marshall County.
This formation affords much valuable material for economic purposes. The upper division furnishes excellent common quarry rock.
The great abundance and variety of its fossils -- crinoids -- now known to be more than three hundred, have justly attracted the attention of geologists in all parts of the world.
The only remains of vertebrates discovered in this formation are those of fishes, and consist of teeth and spines; bone of bony fishes, like those most common at the present day, are found in these rocks. On Buffington Creek, in Louisa County, is a stratum in an exposure so fully charged with these remains that it might with propriety be called bone breccia.
Remains of articulates are rare in this formation. So far as yet discovered, they are confined to two species of tribolites (sic) of the genus phillipsia.
Fossil shells are very common.
The two lowest classes of sub-kingdom radiata are represented in the genera zaphrentis, amplexus and syringapora, while the highest class -- echinoderms -- are found in most extraordinary profusion.
The Keokuk Limestone.--It is only in the four counties of Lee, Van Buren, Henry and Des Moines that this formation is to be seen.
In some localities the upper silicious portion of this formation is known as the Geode bed. It is not recognizable in the northern portion of the formation, nor in connection with it where it is exposed, about eighty miles below Keokuk.
The geodes of the Geode bed are more or less spherical masses of silex, usually hollow and lined with crystals of quartz. The outer crust is rough and unsightly, but the crystals which stud the interior are often very beautiful. They vary in size from the size of a walnut to a foot in diameter.
The economic value of this formation is very great. Large quantities of its stone have been used in the finest structures in the State, among which are the post offices at Dubuque and Des Moines. The principal quarries are along the banks of the Mississippi, from Keokuk to Nauvoo.
The only vertebrate fossils found in the formation are fishes, all belonging to the order selachians, some of which indicate that their owners reached a length of twenty-five or thirty feet.
Of the articulates, only two species of the genus phillipsia have been found in this formation.
Of the mollusks, no cephalopods have yet been recognized in this formation in this State; gasteropods are rare; brachiopods and polyzoans are quite abundant.
Of radiates, corals of general zaphrentes, emplexus and aulopera are found, but crinoids are most abundant.
Of the low forms of animal life, the protozoans, a small fossill related to the sponges, is found in this formation in small numbers.
The St. Louis Limestone.--This is the uppermost of the subcarboniferous group in Iowa. The superficial area it occupies is comparatively small, because it consists of long, narrow strips, yet its extent is very great. It is first seen resting on the geode division of the Keokuk limestone, near Keokuk. Proceeding northward, it forms a narrow border along the edge of the coal fields in Lee, Des Moines, Henry, Jefferson, Washington, Keokuk and Mahaska Counties. It is then lost of sight until it appears again in the banks of Boone River, where it again passes out of sight under the coal measures until it is next seen in the banks of the Des Moines, near Fort Dodge. As it exists in Iowa, it consists of three tolerably distinct subdivisions -- the magnesian, arenaceous and calcareous.
The upper division furnishes excellent material for quicklime, and when quarries are well opened, as in the northwestern part of Van Buren County, large blocks are obtained. The sandstone, or middle division, is of little economic value. The lower or magnesian division furnishes a valuable and durable stone, exposures of which are found on Lick Creek, in Van Buren County, and on Long Creek, seven miles west of Burlington.
Of the fossils of this formation, the vertebrates are represented only by the remains of fish, belonging to the two orders, selachians and ganoids. The articluates are represented by one species of the trilobite, genus phillipsia, and two ostracoid, genera, eythre and beyricia. The mollusks distinguish this formation more than any other branch of the animal kingdom. Radiates are exceedingly rare, showing a marked contrast between this formation and the two preceding it.
The rocks of the subcarboniferous period have in other countries, and in other parts of our own country, furnished valuable minerals, and even coal, but in Iowa the economic value is confined to its stone alone.
The Lower Silurian, Upper Silurian and Devonian rocks of Iowa are largely composed of limestone. Magnesia also enters largely into the subcarboniferous group. With the completion of the St. Louis limestone, the production of the magnesian limestone seems to have ceased among the rocks of Iowa.
Although the Devonian age has been called the age of fishes, yet so far as Iowa is concerned, the rocks of no period can compare with the subcarboniferous in the abundance and variety of the fish remains, and, for this reason, the Burlington and Keokuk limestones will in the future become more famous among geologists, perhaps, than any other formations in North America.
It will be seen that the Chester limestone is omitted from the subcarboniferous group, and which completes the full geological series. It is probable the whole surface of Iowa was above the sea during the time of the formation of the Chester limestone to the southward about one hundred miles.
At the close of the epoch of the Chester limestone, the shallow seas in which the lower coal measures were formed again occupied the land, extending almost as far north as that sea had done in which the Kinderhook beds were formed, and to the northeastward its deposits extend beyond the subcarboniferous groups, outlines of which are found upon the next, or Devonian rock.
The coal-measure group of Iowa is properly divided into three formations, viz., the lower, middle and upper coal measures, each having a vertical thickness of about two hundred feet.
A line drawn upon the map of Iowa as follows, will represent the eastern and northern boundaries of the coal fields in the State: Commencing at the southeast corner of Van Buren County, carry the line to the northeast corner of Jefferson County by a slight easterly curve through the western portions of Lee and Henry Counties. Produce this line until it reaches a point six or eight miles northward from the one last named, and then carry it northwestward, keeping it about the same distance to the northward of Skunk River and its north branch that it had at first, until it reaches the southern boundary of Marshall County, a little west of its center. Then carry it to a point three or four miles northeast from Eldora, in Hardin County; thence westward to a point a little north of Webster City, in Hamilton County; and thence further westward to a point a little north of Fort Dodge, in Webster County.
Lower Coal Measures.--In consequence of the recedence to the southward of the borders of the middle and upper coal measures, the lower coal measures alone exist to the eastward and northward of Des Moines River. They also occupy a large area westward and southward of that river, but their southerly dip passes them below the middle coal measures at no great distance from the river.
No other formation in the whole State possesses the economic value of the lower coal measures. The clay that undrlies almost every bed of coal furnishes a large amount of material for potters' use. The sandstone of these measures is usually soft and unfit, but in some places, as near Red Rock, in Marion County, blocks of large dimensions are obtained which make good building material, samples of which can be seen in the State Arsenal, at Des Moines. On the whole, that portion of the State occupied by the lower coal measures, is not well supplied with stone.
But few fossils have been found in any of the strata of the lower coal measures, but such animal remains as have been found are without exception of marine origin.
Of fossil plants found in these measures, all probably belong to the class acrogens. Specimens of calamites, and several species of ferns, are found in all of the coal measures, but the genus lepidodendron seems not to have existed later than the epoch of the middle coal measures.
Middle Coal Measures.--This formation within the State of Iowa occupies a narrow belt of territory in the southern central portion of the State, embracing a superficial area of about fourteen hundred square miles. The counties more or less underlaid by this formation are Guthrie, Dallas, Polk, Madison, Warren, Clarke, Lucas, Monroe, Wayne and Appanoose.
This formation is composed of alternating beds of clay, sandstone and limestone, the clays or shales constituting the bulk of the formation, the limestone occurring in their banks, the lithological peculiarities of which may offer many contrasts to the limestones of the upper and lower coal measures. The formation is also characterized by regular wave-like undulations, with a parellelism which indicates a widespread disturbance, though no dislocation of the strata have been discovered.
Generally speaking, few species of fossils occur in these beds. Some of the shales and sandstone have afforded a few imperfectly preserved land plants -- three or four species of ferns, belong to the general. Some of the carboniferous shales afford beautiful specimens of what appear to have been sea-weeds. Radiates are represented by corals. The mollusks are most numerously represented. Trilobites and ostracoids are the only remains of known articulates. Vertebrates are only known by the remains of salachians, or sharks, and ganoids.
Upper Coal Measures.--The area occupied by this formation in Iowa is very great, comprising thirteen whole counties, in the southwestern part of the State. It adjoins by its northern and eastern boundaries the area occupied by the middle coal measures.
The prominent lithological features of this formation are its limestones, yet it contains a considerable proportion of shales and sandstones. Although it is known by the name of upper coal measures, it contains but a single bed of coal, and that only about twenty inches in maximum thickness.
The limestone exposed in this formation furnishes good material for building as in Madison and Fremont Counties. The sandstones are quite worthless. No beds of clay for potter's use are found in the whole formation.
The fossils in this formation are much more numerous than in either the middle or lower coal measures. The vertebrates are represented by the fishes of the orders selachians and ganoids. The articulates are represented by the trilobites and ostracoids. Mollusks are represented by the classes cephalopoda, gasteropoda, lamelli, branchiata, brachiapoda and polyzoa. Radiates are more numerous than in the lower and middle coal measures. Protogoans (sic) are represented in the greatest abundance, some layers of limestone being almost entirely composed of their small fusiform shells.
There being no rocks, in Iowa, of permian, triassic or jurassic age, the next strata in the geological series are of the cretaceous age. They are found in the western half of the State, and do not dip, as do all the other formations upon which they rest, to the southward and westward, but have a general dip of their own to the north of westward, which, however, is very slight. Although the actual exposures of cretaceous rocks are few in Iowa, there is reason to believe that nearly all the western half of the State was originally occupied by them; but being very friable, they have been removed by denudation, which has taken place at two separate periods. The first period was during its elevation from the cretaceous sea, and during the long tertiary age that passed between the time of that elevation and the commencement of the glacial epoch. The second period was during the glacial epoch, when the ice produced their entire removal over considerable areas.
It is difficult to indicate the exact boundaries of these rocks; the following will approximate the outlines of the area:
From the northeast corner to the southwest corner of Kossuth County; thence to the southeast corner of Guthrie County; thence to the southeast corner of Cass County; thence to the middle of the south boundary of Montgomery County; thence to the middle of the north boundary of Pottawattamie County; thence to the middle of the south boundary of Woodbury County; thence to Sergeant's bluffs; up the Missouri and Big Sioux Rivers to the northwest corner of the State; eastward along the State line to the place of beginning.
All the cretaceous rocks in Iowa are a part of the same deposits farther up the Missouri River, and in reality form their eastern boundary.
Nishnabotany Sandstone.--This rock has the most easterly and southerly extent of the cretaceous deposits of Iowa, reaching the southeastern part of Guthrie County and the southern part of Montgomery County. To the northward, it passes beneath the Woodbury sandstones and shales, the latter passing beneath the inoceramus, or chalky, beds. This sandstone is, with few exception, almost valueless for economic purposes.
The only fossils found in this formation are a few fragments of angiospermous leaves.
Woodbury Sandstones and Shales.--These strata rest upon the Nishnabotany sandstone, and have not been ovserved outside of Woodbury County, hence their hame. Their principal exposure is at Sergeant's Bluffs, seven miles below Sioux City.
This rock has no value except for purposes of common masonry.
Fossil remains are rare. Detached scales of a lepidoginoid species have been detected, but no other vertebrate remains. Of remains of vegetation, leaves of salix meekii and sassafras cretaceum have been occasionally found.
Inoceramus Beds.--These beds rest upon the Woodbury sandstones and shales. They have not been observed in Iowa, except in the bluffs which border the Big Sioux River in Woodbury and Plymouth Counties. They are composed almost entirely of calcareous material, the upper portion of which is extensively used for lime. No building material is to be obtained from these beds; and the only value they possess, except lime, are the marls, which at some time may be useful on the soil of the adjacent region.
The only vertebrate remains found in the cretaceous rocks are the fishes. Those in the inoceramus beds of Iowa are two species of squolois selachians, or cestratront, and three genera of teliosts. Molluscan remains are rare.
Extensive beds of peat exist in Northern Middle Iowa, which, it is estimated, contain the following areas:
Several other counties contain peat beds, but the character of the peat is inferior to that in the northern part of the State. The character of the peat named is equal to that of Ireland. The beds are of an average depth of four feet. It is estimated that each acre of these beds will furnish two hundred and fifty tons of dry fuel for each foot in depth. At present, owing to the sparseness of the population, this peat is not utilized; but, owing to its great distance from the coal fields and the absence of timber, the time is coming when their value will be realized, and the fact demonstrated that Nature has abundantly compensated the deficiency of other fuel.
The only deposits of the sulphates of the alkaline earths of any economic value in Iowa are those of gypsum at and in the vicinity of Fort Dodge, in Webster County. All others are small and unimportant. The deposit occupies a nearly central position in Webster County, the Des Moines River running nearly centrally through it, along the valley sides of which the gypsum is seen in the form of ordinary rock cliff and ledges, and also occurring abundantly in similar positions along both sides of the valleys of the smaller streams and of the numerous ravines coming into the river valley.
The most northerly known limit of the deposit is at a point near the mouth of Lizard Creek, a tributary of the Des Moines River, and almost adjoining the town of Fort Dodge. The most southerly point at which it has been found exposed is about six miles, by way of the river, from this northerly point before mentioned. Our knowledge of the width of the area occupied by it is limited by the exposures seen in the valleys of the small streams and in the ravines which come into the valley within the distance mentioned. As one goes up these ravines and minor valleys, the gypsum becomes lost beneath the overlying drift. There can be no doubt that the different parts of this deposit, now disconnected by the valleys and ravines having been cut through it, were originally connected as a continuous deposit, and there seems to be as little reason to doubt that the gypsum still extends to considerable distance on each side of the valley of the river beneath the drift which covers the region to a depth of from twenty to sixty feet.
The country round about this region has the prairie surface approximating a general level which is so characteristic of the greater part of the State, and which exists irrespective of the character or geological age of the strata beneath, mainly because the drift is so steep and uniformly distributed that it frequently almost alone gives character to the surface. The valley sides of the Des Moines River, in the vicinity of Fort Dodge, are somewhat abrupt, having a depth there from the general level of the upland of about one hundred and seventy feet, and consequently presents somewhat bold and interesting features in the landscape.
As one walks up and down the creeks and ravines which come into the valley of the Des Moines River there, he sees the gypsum exposed on either side of him, jutting out from beneath the drift in the form of ledges and quarry fronts, having almost the exact appearance of ordinary limestone exposures, so horizontal and regular are its lines of stratification, and so similar in color is it to some varieties of that rock. The principal quarries now opened are on Two Mile Creek, a couple of miles below Fort Dodge.
The reader will please bear in mind that the gypsum of this remarkable deposit does not occur in "heaps" or "nests," as it does in most deposits of gypsum in the States farther eastward, but that it exists here in the form of a regularly stratified, continuous formation, as uniform in texture, color and quality throughout the whole region, and from top to bottom of the deposit as the granite of the Quincy quarries is. Its color is uniform gray, resulting from alternating fine horizontal lines of nearly white, with similar lines of darker shade. The gypsum of the white lines is almost entirely pure, the darker lines containing the impurity. This is at intervals barely sufficient in amount to cause the separation of the mass upon those lines into beds or layers, thus facilitating the quarrying of it into desired shapes. These bedding surfaces have occasionally a clayey feeling to the touch, but there is nowhere any intercalation of clay or other foreign substance in a separate form. The deposit is known to reach a thickness of thirty feet at the quarries referred to, but although it will probably be found to exceed this thickness at some other points, at the natural exposures, it is seldom seen to be more than from ten to twenty feet thick.
Since the drift is usually seen to rest directly upon the gypsum, with nothing intervening, except a a few points where traces appear of an overlying bed of clayey material without doubt of the same age as the gypsum, the latter probably lost something of its thickness by mechanical erosion during the glacial epoch; and it has, doubtless, also suffered some diminution of thickness since then by solution in the waters which constantly percolate through the drift from the surface. The drift of this region being somewhat clayey, particularly in its lower part, it has doubtless served in some degree as a protection against the diminution of the gypsum by solution in consequence of its partial imperviousness to water. If the gypsum had been covered by a deposit of sand instead of the drift clays, it would have no doubt long since disappeared by being dissolved in the water that would have constantly reached it from the surface. Water merely resting upon it would not dissolve it away to any extent, but it rapidly disappears under the action of running water. Where little rills of water at the time of every rain run over the face of an unused quarry, from the surface above it, deep grooves are thereby cut into it, giving it somewhat the sppearance of melting ice around a waterfall. The fact that gypsum is now suffering a constant, but, of course, very slight diminution, is apparent in the fact the springs of the region contain more or less of it in solution in their waters. An analysis of water from one of these springs will be found in Prof. Emery's report.
Besides the clayey beds that are sometimes seen to rest upon the gypsum, there are occasionally others seen beneath them that are also of the same age, and not of the age of the coal-measure strata upon which they rest.
Age of the Gypsum Deposit.--In neither the gypsum nor the associated clays has any trace of any fossil remains been found, nor has any other indication of its geological age been observed, except that which is afforded by its stratigraphical relations; and the most that can be said with certainty is that it is newer than the coal measures, and older than the drift. The indications afforded by the stratigraphical relations of the gypsum deposit of Fort Dodge are, however, of considerable value.
As already shown, it rests in that region directly and unconformably upon the lower coal measures; but going southward from there, the whole series of coal-measure strata from the top of the subcarboniferous group to the upper coal measures, inclusive, can be traced without break or unconformability. The strata of the latter also may be traced in the same manner up into the Permian rocks of Kansas; and through this long series, there is no place or horizon which suggests that the gypsum deposit might belong there.
Again, no Tertiary deposits are known to exist within or near the borders of Iowa to suggest that the gypsum might be of that age; nor are any of the palĉozoic strata newer than the subcarboniferous unconformable upon each other as the other gypsum is unconformable upon the strata beneath it. It therefore seems, in a measure, conclusive, that they gypsum is of Mesozoic age, perhaps older than the Cretaceous.
Lithological Origin.--As little can be said with certainty concerning the lithological origin of this deposit as can be said concerning its geological age, for it seems to present itself in this relation, as in the former one, as an isolated fact. None of the associated strata show any traces of a double decomposition of pre-existing materials, such as some have supposed all deposits of gypsum to have resulted from. No considerable quantities of oxide of iron nor any trace of native sulphur have been found in connection with it; nor has any salt been found in the waters of the region. These substances are common in association with other gypsum deposits, and are regarded by some persons as indicative of the method of or resulting from their origin as such. Throughout the whole region, the Fort Dodge gypsum has the exact appearance of a sedimentary deposit. It is arranged in layers like the regular layers of limestone, and the whole mass, from top to bottom, is traced with fine horizontal laminĉ of alternating white and gray gypsum, parallel with the bedding surfaces of the layers, but the whole so intimately blended as to form a solid mass. The darker lines contain almost all the impurity there is in the gypsum, and that impurity is evidently sedimentary in its character. From these facts, and also from the further one that no trace of fossil remains has been detected in the gypsum, it seems not unreasonable to entertain the opinion that the gypsum of Fort Dodge originated as a chemical precipitation in comparatively still waters which were saturated with sulphate of lime and destitute of life; its stratification and impurities being deposited at the same time as clayey impurities which had been held suspended in the same waters.
Physical Properties.--Much has already been said of the physical properties or character of this gypsum, but as it is so different in some respects from that of other deposits, there are yet other matters worth of mention in connection with those. According to the results of a complete and exhaustive analysis by Prof. Emery, the ordinary gray gypsum contains only about eight per cent. of impurity; and it is possible that the average impurity for the whole deposit will not exceed that proportion, so uniform in quality as it is from to bottom and from one end of the region to the other.
When it is remembered that plaster for agricultural purposes is sometimes prepared from gypsum that contains as much as thirty per cent. of impurity, it will be seen that ours is a very superior article for such purposes. The impurities are also of such a character that they do not in any way interfere with its value for use in the arts. Although the gypsum rock has a gray color, it becomes quite white by grinding, and still whiter by the calcining process necessary in the preparation of plaster of Paris. These tests have all been practically made in the rooms of the Geological Survey, and the quality of the plaster of Paris still further tested by actual use and experiment. No hesitation, therefore, is felt in stating that the Fort Dodge gypsum is of as good a quality as any in the country, even for the finest uses.
In view of the bounteousness of the primitive fertility of our Iowa soils, many persons forget that a time may come when Nature will refuse to respond so generously to our demand as she does now, without an adequate return. Such are apt to say that this vast deposit of gypsum is valueless to our commonwealth, except to the small extent that it may be used in the arts. This is undoubtedly a short-sighted view of the subject, for the time is even now rapidly passing away when a man may purchase a new farm for less money than he can re-fertilize and restore the partially wasted primitive fertility of the one he now occupies. There are farms even now in a large part of the older settled portions of the State that would be greatly benefited by the proper application of plaster, and such areas will continue to increase until it will be difficult to estimate the value of the deposit of gypsum at Fort Dodge. It should be remembered, also, that the inhabitants of an extent of country adjoining our State more than three times as great as its own area will find it more convenient to obtain their supplies from Fort Dodge than from any other source.
For want of direct railroad communication between this region and other parts of the State, the only use yet made of the gypsum by the inhabitants is for the purposes of ordinary building stone. It is so compact that it is found to be comparatively unaffected by the frost, and its ordinary situation in walls of houses is such that it is protected from the dissolving action of water, which can at most reach it only from occasional rains, and the effect of these is too slight to be perceived after the lapse of several years.
One of the citizens of Fort Dodge, Hon. John F. Duncombe, built a large, fine residence of it, in 1861, the walls of which appear as unaffected by exposure and as beautiful as they were when first erected. It has been so long and successfully used for building stone by the inhabitants that they now prefer it to the limestone of good quality, which also exists in the immediate vicinity. This preference is due to the cheapness of the gypsum, as compared with the stone. The cheapness of the former is largely due to the facility with which is quarried and wrought. Several other houses have been constructed of it in Fort Dodge, including the depot building of the Dubuque & Sioux City Railroad. The company have also constructed a large culvert of the same material to span a creek near the town, limestone only being used for the lower courses, which come in contact with the water. It is a fine arch, each stone of gypsum being nicely hewn, and it will doubtless prove a very durable one. Many of the sidewalks in the town are made of the slabs or flags of gypsum which occur in some of the quarries in the form of thin layers. They are more durable than their softness would lead one to suppose. They also possess an advantage over stone in not becoming slippery when worn.
The method adopted in quarrying and dressing the blocks of gypsum is peculiar, and quite unlike that adopted in similar treatment of ordinary stone. Taking a stout auger-bit of an ordinary brace, such as is used by carpenters, and filing the cutting parts of it into a peculiar form, the quarryman bores his holes into the gypsum quarry for blasting, in the same manner and with as great facility as a carpenter would bore hard wood. The pieces being loosened by blasting, they are broken up with sledges into convenient sizes, or hewn into the desired shapes by means of hatchets or ordinary chopping axes, or cut by means of ordinary wood-saws. So little grit does the gypsum contain that these tools, made for working wood, are found to be better adapted for working the former substance than those tools are which are universally used for working stone.
Besides the great gypsum deposit of Fort Dodge, sulphate of lime in the various forms of fibrous gypsum, selenite, and small, amorphous masses, has also been discovered in various formations in different parts of the State, including the coal measure shales near Fort Dodge, where it exists in small quantities, quite independently of the great gypsum deposit there. The quantity of gypsum in these minor deposits is always too small to be of any practical value, and frequently minute. They usually occur in shales and shaly clays, associated with strata that contain more or less sulphuret of iron (iron pyrites). Gypsum has thus been detected in the coal measures, the St. Louis limestone, the cretaceous strata, and also in the lead caves of Dubuque. In most of these cases it is evidently the result of double decomposition of iron pyrites and carbonate of lime, previously existing there; in which cases the gypsum is of course not an original deposit as the great one at Fort Dodge is supposed to be.
The existence of these comparatively minute quantities of gypsum in the shales of the coal measures and the subcarboniferous limestone which are exposed within the region of and occupy a stratigraphical position beneath the great gypsum deposits, suggests the possibility that the former may have originated as a precipitate from percolating waters, holding gypsum in solution which they had derived from that deposit in passing over or through it. Since, however, the same substance is found in similar small quantities and under similar conditions in regions where they could have had no possible connection with that deposit, it is believed that none of those mentioned have necessarily originated from it, not even those that are found in close proximity to it.
The gypsum found in the lead caves is usually in the form of efflorescent fibers, and is always small in quantity. In the lower coal-measure shale near Fort Dodge, a small mass was found in the form of an intercalated layer, which had a distinct fibrous structure, the fibers being perpendicular to the plane of the layer. The same mass had also distinct, horizontal planes of cleavage at right angles with the perpendicular fibers. Thus, being more or less transparent, the mass combined the characters of both fibrous gypsum and selenite. No anhydrous sulphate of lime (anhydrite) has been found in connection with the great gypsum deposit, nor elsewhere in Iowa, so far as yet known.
The only locality at which this interesting mineral has yet been found in Iowa, or, so far as is known, in the great valley of the Mississippi, is at Fort Dodge. It occurs there in very small quantity in both the shales of the lower coal measures and in the clays that overlie the gypsum deposit, and which are regarded as of the same age with it. The first is just below the city, neer Rees' coal bank, and occurs as a layer intercalated among the coal measure shales, amounting in quantity to only a few hundred pounds' weight. The mineral is fibrous and crystalline, the fibers being perpendicular to the plane of the layer. Breaking also with more or less distict horizontal planes of cleavage, it resembles, in physical character, the layer of fibro-crystalline gypsum before mentioned. Its color is light blue, is transparent and shows crystaline facets upon both the upper and under surfaces of the layer; those of the upper surface being smallest and most numerous. It breaks up readily into small masses along the lines of the perpendicular fibers or columns. The layer is probably not more than a rod in extent in any direction [ed. note: a rod is 16.5 feet in length] and about three inches in maximum thickness. Apparent lines of stratification occur in it, corresponding with those of the shales which imbed it.
The other deposit was still smaller in amount, and occurred as a mass of crystals imbedded in the clays that overlie the gypsum at Cummins' quarry in the valley of Soldier Creek, upon the north side of town. The mineral is in this case nearly colorless, and but for the form of the separate crystals would closely resemble masses of impure salt. The crystals are so closely aggregated that they enclose but little impurity in the mass, but in almost all cases their fundamental forms are obscured. This mineral has almost no real practical value, and its occurrence, as described, is interesting only as a mineralogical fact.
This mineral has been found only in minute quantities in Iowa. It has been detected in the coal-measure shales of Decatur, Madison and Marion Counties, the Devonian limestone of Johnson and Bremer Counties and in the lead caves of Dubuque. In all these cases, it is in the form of crystals or small crystalline masses.
Epsomite, or native epsom salts, having been discovered near Burlington, we have thus recognized in Iowa all the sulphates of the alkaline earths of natural origin; all of them, except the sulphate of lime, being in very small quantity. Even if the sulphate of magnesia were produced in nature, in large quantities, it is so very soluble that it can accumulate only in such positions as afford it complete shelter from the rains or running water. The epsomite mentioned was found beneath the overhanging cliff of Burlington limestone, near Starr's mill, which are represented in the sketch upon another page, illustrating the subcarboniferous rocks. It occurs in the form of efflorescent encrustations upon the surface of stones and in similar small fragile masses among the fine debris that has fallen down beneath the overhanging cliff. The projection of the cliff over the perpendicular face of the strata beneath amounts to near twenty feet at the point where epsomite was found. Consequently the rains never reach far beneath it from any quarter. The rock upon which the epsomite accumulates is an impure limestone, containing also some carbonate of magnesia, together with a small proportion of iron pyrites in a finely divided condition. It is doubtless by double decomposition of these that the epsomite results. By experiments with this native salt in the office of the Survey, a fine article of epsom salts was produced, but the quantity that might be annually obtained there would amount to only a few pounds, and of course is of no practical value whatever, on account of its cheapness in the market.
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