NOTES ON THE GEOLOGY OF JOHNSON COUNTY, IOWA. / BY CLEMENT L. WEBSTER.1 (Extracted from the May number of the American Naturalist.) rpHE superficial geology, or the study of the nature, origin, and distribution of the loose materials, or superficial detritus commonly denominated loess, drift, and aluvium, constitutes one of the most interesting as well as important fields for study that fall within the scope of investigation of the geologist. The interpretation of the phenomena of the glacial period, the cause of the advance and retreat of the ice sheets, the condition of the climate and of the fauna and flora of the surface (now covered by drift) during the interglacial time, the extent and condition of 1 State University, Iowa. Geology of Johnson County. 409 the preglacial river channels which have become obliterated, at least in part, and the condition of things during the Champlain period, are all subjects of the most profound interest to the geologist. In this paper I have endeavored to elucidate some of these phe- nomena as observed in Iowa, and it is hoped that it may not be without at least some value to the broad field of science. In the hard Devonian limestone in the west bank of the Iowa River, at Iowa City, are numerous (often large) very interesting potholes, which have been formed by the action of running water; and as they shed so much light upon the condition of things at this locality at the beginning of the Carboniferous age, a short descrip- tion of a few of the more prominent ones here is deemed not to be without interest and value. The rocks in which these potholes are formed, rises to a height of from twenty to forty feet above the water in the Iowa River. Pothole number one: Circular, sides smooth and symmetrical, sloping gradually to the bottom, upper margin rounded, depth five feet, diameter at the top two feet, and at the bottom one foot. Pothole No. 1. The following diagram will illustrate this description :- One foot from this is another smaller subcircular pothole, having a depth of two feet and a diameter at the top of one foot, and at the 410 Geology of Johnson County. bottom of ten inches, with unsymmetrical but smooth sides; upper margin smooth and somewhat flaring. About half way from the top to the bottom, cavities have been formed in the sides by the action of the water which formed the pothole. The upper margin of this pothole has a small channel cut through to the east, apparently by the overflow. Pothole number two: This is one of the largest and most inter- esting examples to be found here. This pothole has been formed by the union of several minor ones. Its longest diameter is from east to west, somewhat subovate in marginal outline, sides unsym- metrical but smooth, more or less gently sloping to the bottom, upper margins unevenly rounded, depth seven feet, length from east to west seven feet, and width of the eastern portion at the top two and one-half feet, and marginal width of the western portion four and one-half feet, contracting at the bottom to a width of two and one-half feet. In the west side of this pothole, about midway from top to bottom, large, more or less ramifying cavities, varying from three .feet to eight feet in extent, have been worn by the action of the water which formed the pothole. The overflow to the east has cut a channel about three feet in depth and two feeet in width. About four feet to the right of the foregoing is a small bowl-shaped depression, with smooth symmetrical sides, and rounded or flaring margin. Near this is another smaller saucer-shaped depression, having a diameter of sixteen inches, and a depth of six inches. These are beautiful examples of how some of these potholes were begun. The following cut (from a photograph) will illustrate the form and position of these depressions in relation to the large pothole above described:- Pothole number three : Subcircular in outline, sides somewhat irregular, smooth, upper margin slightly flaring. For the first four feet the sides are nearly perpendicular; below this point the hole rapidly enlarges to the east, but soon contracts, and continues its downward course in the form of a gradually-narrowing oblique fissure, that finally unites with a rather large, more or less horizon- tal, crevice between the beds of rock.1 1 Thia fissure is filled with a black, combustible, carbonaceous mud of bituminous matter, which is usually underlayed and sometimes over* layed by a reddish-brown clay. Geology of Johnson County. 411 The following diagram will illustrate this description :- Pothole number four: This is the largest and deepest of any yet Pothole No. 3. a a, coal seam. observed, having a depth of slightly upward of fourteen feet. Although much of the rock in which this pothole was formed has aa, coal seam; the lower part shaly and containing fish teeth; b b, ash-colored and greenish underclay; cc, coarse sandstone in curved laminre. been removed by quarrying, yet enough of the excavation itself remains to demonstrate the fact that it was formed by the enlarge- 412 Geology of Johnson County. ment of a deep vertical fissure. In this same rock are numerous other minor potholes of much interest, many of them showing unmistakable evidence of having originated in a larger or smaller vertical fissure, their upper margins being invariably rounded, or more or less strongly flaring, with often a larger or smaller channel cut through to the east by the overflow, thus demonstrating the fact that the flow of water which formed them was from the west. Many of these potholes are filled (and all showing evidence of having been originally) with a hard, stiff, reddish-brown, sometimes brownish-black, clay, similar to the underclay of coal seams. Not only are these potholes filled by this clay, but also the numerous vertical and horizontal fissures and spaces between the rock bed- ding filled with the same material. This reddish-brown color, however, is probably due to the infiltration from the, in places, very ferruginous drift material, which immediately overlies them. About thirty years ago, during the geological survey of Iowa by James Hall, important facts (since mostly obliterated by the exten- sive quarrying of the rock) were observed in regard to this phe- nomena ; and for the sake of greater clearness in the matter, I here give a partial compilation of the description of it as found in the survey:-1 " In a cliff of limestone of the Hamilton Group, at Iowa City, the following phenomena is observed: - "Beneath beds of nearly horizontal limestone appears a black band extending thirty or forty feet: this consists of black carbonaceous mud, the upper part having the character of cannel coal, and the lower part a slaty carbonaceous shale. " Beneath this, and less extended, a thicker layer of greenish- grey clay, of the character of underclay of coal seams, fills the upper and broader part of the cavity ; while below this, and occu- pying the deepest parts, is a coarse sandstone, which follows, in its line of lamination, the curvature of the limestone upon which it lies." This description is illustrated by the following diagram :- " Here we have all the phenomena attending a true coal-measure seam of coal: the sandstone, the underclay, and the coal seam rest- ing upon it; and to complete the analogy, the slaty portion of the seam contains fish teeth of carboniferous character. All this is * Hall's Geological Survey of Iowa, Vol. I., Part 1, pp. 129-133 and 265. Geology of Johnson County. 413 enclosed in limestone, which, in the State of New York, where the series is more complete, lies at a depth of more than five thousand feet below the coal measures." The result of the most critical examination and study of the phenomena as observed to-day, is such as to corroborate the state- ment then made in regard to this subject, which is as follows:- "The coarse and fine sand were first transported, and, entering some fissure [probably a pothole] in the rock, continued in deposi- tion in this cavity, while a bed of similar sandstone was being formed outside upon the bottom of the sea. This ceased, and then came the clay, which continued in like manner, while the under clay of an exterior coal bed was in process of deposition. " Lastly, the carboniferous mud, derived from the material of a eoal seam, was filtered through the fissure, filling the remaining space, and spread out in the narrow seam beyond. There is no mingling of the material, as if resulting from the breaking up of a coal seam at a later and modern period. " Every part is as distinct as in the coal measures elsewhere; and this only could have resulted from a participation in the cause then operating to produce those extensive beds of sand, shale, clay, and coal which make up the coal measures. This point is near the northeast margin of the coal basin, and beyond the limits of any productive coal seams; a few isolated patches of sandstone and shale being all the remaining evidence of the extension of the series in that vicinity." It is believed that the immediate valley or channel of the Iowa River, from Coalville to Iowa City, and perhaps north from this point, in Johnson County, represents the channel of an old Devo- nian stream. Occupying a position in the east side of the valley of the Iowa River, where an abrupt curve is made, near Iowa City, is a patch of soft, friable carboniferous sandstone, having a length of one- fourth of a mile, and a breadth of one-eighth of a mile or less. This sandstone occupies a valley of erosion in the Devonian lime- stone, and which has a depth of between thirty and forty feet. That this was a valley of erosion, formed by the action of flowing water prior to the time when the sandstone was deposited, is obvious, as the sides of this valley are seen, after the removal of the sandstone, to be smoothed and worn by the long-continued action of running water. 414 Geology of Johnson County. The channel of the Iowa Kiver, from Iowa City north, in this county, has been eroded to a depth of from twenty feet to upwards of one hundred feet into the hard Devonian limestone. As I have before intimated, it is believed that this valley was formed, to a great extent at least, by an old Devonian stream, and then sub- sequently filled by the sandstone during the Carboniferous age. The glacial drift in Johnson County, so far as it has been observed, is everywhere covered by a profound mantle of loess, so that it can be observed only along the borders of streams, and along the axis of surface drainage, where the overlying formation has been cut through, thus exposing the drift at the bottom. This formation, so far as can be made out, attains a thickness of from four feet to eighteen feet, and is composed of clay, sand, gravel, and boulders. The boulders, however, are rarely observed, except at the bottom of ravines, where they are sometimes quite numerous, and vary in weight from ten pounds to upwards of two thousand pounds. The drift, for the most part, shows but slight evidence of modification or rearrangement of its materials. In some places, however, it contains an abundance of ferruginous matter, so much indeed as to give it, at limited localities, a deep-red color. The gravel of this formation is made up of more or less rounded frag- ments of Devonian limestone, derived from the subjacent strata, smoothed and well rounded, often beautiful striated, pebbles of granite, quartz, trap, greenstone, and others of igneous origin, which have been derived from the north. The boulders are also of the same material and origin with the exception of those of Sioux quartzite, which were derived from the northwestern portion of the State. Devonian fossils (and rarely Carboniferous), derived from the underlying rock, are also common. The old forest bed is also well represented, and occupies, as is usual throughout other portions of the State, a low horizon in the drift formation. While digging a well on the farm of Mr. Joseph Hedger, about five miles southeast from Iowa City, the forest bed was struck at a depth of about twenty-eight feet. This formation was represented by a dark-brown, slightly-combustible peat formation, which was overlayed by, and slightly mixed with, a layer of coniferous wood and twigs. This peat also contained abundant remains of plants, well-preserved seeds (apparently those of grass), and abundant remains of Coleopterous insects. This bed rests directly upon hard, Geology of Johnson County. 415 stiff, distinctly stratified blue clay. This formation has been struck at numerous localities at a depth of from twenty-eight feet to sixty feet below the surface, in the central portion of this county. A usual feature of the peat division of the forest bed of this region are the remains of Coleopterous insects. The peat at all these localities was evidently formed where it is now found, and are parts of one and the same bed. A limited deposit of similar ancient peat was also discovered in Adair County, one hundred and seventy miles to the westward of Iowa City, on section twenty-two, township seventy-five, range thirty-two. This bed was found to be between two and three feet in thickness. The drift, apparently only slightly if at all modified, rested directly upon it, and it was underlayed by a dull, bluish- clayey bed. At another locality, near Davenport, fifty miles east of IowTa City, a deposit of peat occurs almost upon the very brow of the bluffs that border the valley of the Mississippi. This example is one of unusual interest, in consequence of the existence there of an extensive bed of ancient peat which is covered to the depth of several feet beneath the prairie soil, and the discovery in the clay, above the peat, of the remains of a mammoth. The following section, compiled from White's Geological Survey of Iowa, vol I., part i., pp. 119, 120, will show the position of the deposit in relation to the drift:- " No. 1. The ordinary prairie soil, one foot. The prairie here extends to the edge of the bluff, gently sloping backward toward the north. "No. 2. The 'yellow clay' or loam, twenty feet thick, iron- stained, frequently distinctly laminated; laminae curved, and have their layers of sand interstratified in some places. It contains small calcareous nodules and shells of the genera Succinea, Helicina and Pupa. "No. 3. Bluish-grey clay, three to five feet thick, not stratified; contains a few shells like those of No. 2. A tusk, several teeth, and some other portions of Elephas primigenius (?) were found, just at the junction of Nos. 2 and 3. " No. 4. A bed of brown peat one foot thick, which burns toler- ably well. In some places the peat moss, Hypwm aduncum, was so well preserved as to be recognized. Quantities of much-decomposed coniferous wood are distributed throughout this bed. 416 Geology of Johnson County. " No. 5. Ancient soil, two feet thick, very dark loam, resembling the peat, but more decomposed. Contains no shells or other fossil remains. " No. 6. Blue clay, very tenacious, containing sand, gravel, and small boulders; pebbles and boulders, all water-worn, and many of them distinctly glacier scratched. Thickness unknown." " The exposure was made by the excavation of the Chicago, Rock Island and Pacific Railroad Company, previous to which there was no appearance at the surface to indicate the presence of anything more than the ordinary drift deposit." Bed No. 5 of this series is undoubtedly only a more thoroughly decomposed and finely comminuted portion of bed No. 4. This bed of peat, also, like that at Iowa City, contains remains of Coleopterous insects. While boring a well on the northeast quarter of section ten, township eighteen, range five, in Linn County, a deposit of peat and coniferous wood four feet in thickness was struck at a depth of ninety-nine feet below the surface. From this well, into which a tight galvanized iron tubing has been forced, escapes a constant supply of natural, combustible gas (the peat probably being the origin of it), but whether of sufficient quantity to be of practical value, is a question to be answered by investigation. A few miles from this locality another well of the same character is reported. In the northern portion of the State the peat formation is seldom, if ever, observed as a member of the forest bed. These beds of peat are of interglacial origin, and was coexistent with the luxuriant forests of conifers which, in interglacial times, covered the surface of what is now known as Iowa.1 The occurrence also of the well-preserved seeds of plants and the abund- ant remains of insects in this formation are features of pecu- liar interest. A critical and somewhat extended study of the forest bed and other superficial formations in different portions of the State, reveals facts which seem to substantiate the theory advanced in regard to the relative age of the peat formations. The topography of the surface of Johnson County is, for the most part, peculiarly that of loess regions, being more or less broken and rolling, the elevations having rounded tops, and more or 1 These beds of peat were believed by Dr. White not to be of interglacial origin. White's Geological Survey of Iowa, vol. i., p. 121. Geology of Johnson County. 417 less rapidly ascending sides, and rising to a height of from ten feet to upward of sixty feet above the intervening depressions. The valleys of the Iowa River and its tributaries are relatively narrow and deep, and bordered by more or less steep acclivities, and flanked at frequent intervals by deep but narrow and rapidly ascending ravines, and these again often ramify in shallow depres- sions which draw the water quite effectually from nearly all portions of the upland. The immediate valley of the Iowa River, as I have before inti- mated, attains an average width of one-half of a mile, and a depth below its immediate borders of from sixty feet to upward of one hundred and seventy-five feet. Well-defined terraces are sometimes observed along this stream, and they have been produced by the deepening of the valley by the action of the waters of the stream. As has been before stated, the surface of nearly, if not quite all of this county, is occupied by a loess formation, which effectually conceals the underlying drift formation, except along the valley- sides and the axis of surface drainage, where the overlying deposit has been qut through. This deposit attains a greater thickness along the streams than adjacent to them, and consists, for the most part, of an exceedingly finely comminuted yellow or buff-colored clayey earth, with an admixture of humus in favorable situations, as in the beds of drainage depressions and in the valleys, as well as most usually the more level surface of the upland, which gives to it a color varying from a light-ash to a deep-black. Upon the higher points, however, the soil usually contains comparatively little humus, for the reason that it is swept down by the rains as last as it is accumulated by the decay of vegetation, and deposited in the beds of ravines and the slight valley bottoms. The following section, taken at the "brick-yard," in the north- east portion of Iowa City, gave the following result:- 1. Very fine brownish "loamy" soil, containing humus-three inches. 2. Very fine and homogeneous yellow-clayey earth-fifteen feet. 3. Very fine and homogeneous bluish-grey, clayey earth, having more or less numerous brownish-drab streaks running through it, and containing numerous fossils, many of which were in a crushed condition-five feet. Entire thickness unknown. The different divisions of this section pass into each other by 418 Geology of Johnson County. very gradual and imperceptible gradations. In a railroad cut, about one and one-fourth miles west of Iowa City, the loess is seen to attain a thickness of twenty-three feet and five inches. The humus-stained division, No. 1, attains a thickness of five inches; while the second, or yellow division, attains a thickness of about twelve feet; and below this, the third, or blue division, which attains a thickness of eleven feet, and rests immediately upon the drift. This portion contains an abundance of fossils, most of which, however, are in a better preserved condition than those of its equivalent at the " brick-yard." One mile south of Iowa City the loess is seen to attain a thick- ness of twenty feet, and is very fine and homogeneous throughout, being of a yellow or yellowish-buff color, with the exception of a humus-stained layer, of a few inches in thickness at the top. The yellow clay at this locality rests directly upon the drift, the blue division being absent, as also fossil remains, so far as observed. At Oxford, in the extreme western portion of the county, the typical yellow loess is seen to have a thickness of from fifteen feet to twenty-two feet, the blue division, however, not being present. No fossils were observed in the loess in this region. Occupying the same relative position to the loess, and presenting the same general character as at other described localities, is the drift forma- tion. At this locality, as well as several others in the county, the loess is seen to contain more or less extensive accumulations of fine silicious sand; also, at two or three places, I observed small accu- mulations, or pockets, of well-rounded and smoothed quartz and greenstone pebbles of drift origin. This material may have been derived from detritus ladened ice, floating from the north and dropping its burden while this formation was in process of deposi- tion. At numerous localities the loess contains abundant calcareous concretions and ferruginous tubules of various dimensions, while at other localities it is devoid of them. At Solon, in the northeast part of the county, the loess is some- what thinner than at the previously-described localities, but is essentially the same, although the lower, blue, division is not noticed, and the upper, black, humus-stained layer, is somewhat thicker, owing to the somewhat less broken surface, and the lessened facility with which the surface is " washed " by rains, thus remov- ing much of the humus accumulated by decomposing vegetation. Geology of Johnson County. 419 Below I give a catalogue of all the fossils yet obtained from the loess of Johnson County; doubtless, however, many others existed which have not as yet been discovered.1 Those marked * do not occur as living forms in the county at the present time :- Zonites viridulus (Mke.). Zonites limaiulus * (Ward). Zonites fulvus (Drap.). Patula strigosa * (Gid.). Patula striatella (Say). Ferussacia subcylindrica (L.). Pupa muscorum* (L.). Pupa blandi * (Mone.). Vertigo simplex* (Gid.). Mesodon multilineata (Say). Vallonia pulchella (Mull.). Succinea ava.ra (Say). Succinea avara var vermeta,- Succinea obliqua (Say). Plelicina occulta (Say). Limnoea desidiosa (Say). Physa-sp. (?) Pmdnwn-sp. (?) Egg-shell of some small Helix. The loess formation constitutes a prominent feature of the super- ficial geology of Iowa, being developed to a greater or less extent over a large portion of the State. It is believed that the material of this formation was deposited during the Champlain period; and facts seem to demonstrate, moreover, that this was not only a period of somewhat lower level in Iowa as well as other places, than the present, but also that the amount of depression increased somewhat to the northward, so that the streams flowing to the southward had usually a diminished slope, with a consequently slackened flow of the waters, and many greater or less expansions along their course, and from these silt-ladened paters the material of the loess forma- tion was derived. 1 For this catalogue of species I am much indebted to Professor B. Shimick, of Iowa City. AU the species listed have been personally collected by him.