One cannot work in the Iowan Drift area and in adjacent areas of older drift without confronting the problem of the loess. During the field seasons of 1914 and 1915 while associated with Dr. Wm. C. Alden of the U. S. Geological Survey in reviewing the field evidences for and against an Iowan stage of glaciation, the writer became interested in certain phases of the loess and their interpretations. Through the kindness of his senior colleague, the writer has the privilege of briefly discussing these phases before the Iowa Academy of Science. A more complete discussion will appear in the forthcoming report of the Iowa Geological Survey in connection with the report on the investigations of the Iowan Drift, under the joint authorship of Doctor Alden and the writer

Leighton, M. M.

English

University of Northern Iowa

Proceedings of the Iowa Academy of Science Volume 24 Annual Issue Article 15 1917 The Iowan Glaciation and the So-Called Iowan Loess Deposits M. M. Leighton University of Washington Copyright ©1917 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Leighton, M. M. (1917) "The Iowan Glaciation and the So-Called Iowan Loess Deposits," Proceedings of the Iowa Academy of Science, 24(1), 87-92. Available at: https://scholarworks.uni.edu/pias/vol24/iss1/15 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. THE IOWAN GLACIATIO:N AND THE SO-CALLED row AN LOE SS DEPOSITS. M. M. LE~IGHTON. One cannot work in the Iowan Drift area and in adjacent areas of older drift without confronting the problem of the loess. During the field seasons of 1914 and 1915 ·while asso-ciated with Dr. Wm. C. Alden of the U. S. Geological Survey in reviewing the field evidences for and against an Iowan stage of glaciation, the writer became interested in eertain 1phases of the loess and their interpretations. Through the kindness of his senior colleague, the writer has the privilege of briefly dis-cussing these phases before the Iowa Academy of Science. A more complete discussion will appear in the forthcoming re-. port of the Iowa Geological Survey in connection with the re-port on the investigations of the Iowan Drift, under the joint authorship of Doctor Alden and the writer. l. The Weathering of the Loess.-In the various papers which have appeared on the loess, little attention has been de-voted to the weathering of the loess. ·what will be said here will concern only that loess which is assoeiated with the Iowan drift-sheet. From a careful examination of the exposures of the loess, it has become clear to the writer that it has been partly leached of its calcareous material and oxidized to some depth since it was deposited. The features of the average ex-posure where eight feet or more arc shown arc as follows: FEET 3. Soil, black, changing below to dirty brown, no pebbles; usual thickness ..................... 112-ll;\i 2. Leached loess, buff to yellow, does not react to dilute hydrochloric a.cid, no fossils or lime con-cretions; thickness (in some cases as much as 12 feet) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8 1. Calcareous loess, usually of lighter color than the noncalcareous except where grayish; the grayish color may not appear for several feet below the top of the calcareous zone; snail fossils com-monly pre.sent, also lime concreti.ons. This zone is usually not shown in cuts less than 8 feet deep. 1Leighton: The Iowan Glaciation and the So-Called Iowan Loess DepositsPublished by UNI ScholarWorks, 191788 IOWA ACADEMY OF SCIENCE VoL. XXIV, 1917 The persistency of these phenomena in cut after cut makes them of significance in reading the history recorded by the loess. The buff loess and the gray loess have heretofore been generally regarded as separate and distinct deposits, differing considerably in age. The buff loess has been thought to be of Iowan age and younger, while the gray has been held to be of approximately Kansan age, it being held that the gray prob-ably has the same relation to the Kansan drift as the buff seems to have to the Iowan drift. These conclusions, however, do not seem to the writer to be well based. Of the many exposures examined on the east and south sides of the Iowan area, there was not a single one which showed good evidence of an interval of time between the depo-sition of the gray and the huff. There is no zone of leaching at the top of the gray, sneh as would be expected if the two deposits lielong to two different c1pochs, nor is there any other strong evidence of weathering rffec.ts that would differentiate the two. Indeed there is a continuance of lime carbonate par-ticles and shells from the gray UP' into the buff, and in most cases there is a transition in color. It is true that in some instances rusty streaks occur at the contact, but these were found also indiscriminately at any horizon. In view of these features of gradation and the ahsence of any that distinctly separate the two in terms of time intervals, the writer has become convinced that the two are of the same geological age, that the mass of the loess was originally gray, and that the buff is to be regarded merely as the oxidized phase of the gray. Another important historical point to be noted from a study of the weathering of the loess is that the leached zone records the fact that the loess has been subjected to the solvent action of ground water. for a sufficient length of time for the cal-careous particles and snail shells to be dissolved out to a depth of several feet. Fossils are not seen in many shallow cuts for they 00cur only in the zone which has not been leached. One must bear in mind, however, the possibility that the upper part of the loess was deposited as a noncalcarcous clay. This may be true of some of the leached zone, but it would be an extreme view to assume that the ground water has performed no work since the loess was deposited, especially if the mass of the loess is pre-Wisconsin in age as will be shown later. But aside 2Proceedings of the Iowa Academy of Science, Vol. 24 [1917], No. 1, Art. 15https://scholarworks.uni.edu/pias/vol24/iss1/15THE IOWAN GLACIATION 89 from these reasonable grounds, there is good basis for think-ing that the noncalcareous zone of the loess is largely due to the leaching process. There are to be found on the surface today living snails, which, according to Professor Shimek, who has made an extensive study of them, are mostly of the same species as the snails whose shells arc found as fossils in the loess. If this is true, it is quite evident that if the loess has suffered no leaching, snail shells should be found in the non-calcareous zone as well as in the calcareous ~one beneath. As to where present snails secure their carbonate of lime, it is quite possible that the lime may be obtained from the feldspar ~Jarticles of the clay, which then may be carbonated by ground water. Another evidence that the loess has in reality suffered leaehing is the fact that lime concretions are found near the top of the c;lcareous zone, the material of these concretions being that which was dissolved from the zone above. These phenomena, therefore, lead to the conclusion that the mass of the loess, which is associated with the Iowan drift, was once calcareous and gray, and that it accumulated at a time when the rate of deposition was greater than the rate of leaching. This suggests strongly a special episode of deposi-tion for this particular loess. 2. The Source of the Loess.-The loess associated with the Iowan area is thickest in three situations: (1) along the river valleys leading from the Iowan drift ; ( 2) aroun.d the border of the Iowan drift; and (3) in those elliptical hills which McGee termed paha. In some instances the buff loess is so thick along the larger valleys that the valley walls rise distinctly above the border-ing upland like low ridges. This is so noticeable along the Cedar river southwest of Marion and along the W apsipinicon river west of Oxford Junction that one is reminded of the com-mon remark of the early geologists who studied these localities, that the streams left the plains to cut through the hills.1 Of course, in some instances dune sand is associated. In many other places along valleys the loess is thick without forming distinct ridges. The accumulation of loess in such marked de-posits afong valleys is probably due to nearness to a source 1u Is evident in such cases that the ridges came into existence after the valleys were cut. 3Leighton: The Iowan Glaciation and the So-Called Iowan Loess DepositsPublished by UNI ScholarWorks, 191790 IOWA ACADEMY OF SCIENCE VoL. XXIV, 1917 of supply, namely the valley flats, and to the vegetation and topography favoring lodgment, all of which has been pointed out by Professor Shimek. The notable thickness of the loess around the border of the Iowan drift is too well kno\Yn to demand much further refer-ence. The fact that it gradually diminishes in thickness and become.s finer as distance increases from the Iowan border, to-gether with the fact that it is thick even away from valleys, indieate that the Iowan area also was a source of supply. With-in the Iowan area itself the locss is generally absent or nearly so, but to this there arc exceptions as would be expected. It ii? obvious that deposits would be made wherever there were ob-structions. Some such conditions gave rise to the 1paha, the wind depositing dust and sand about rock projections, glacial drumlins or even sand dunes. Their persistent southeasterly trend probably is best accounted for by assuming either that the winds were prevailing northwesterlies or that the drumlins trended in that direction. If the source of the loess was the Iowan drift and the valley flats, and if the mass of the loess was deposited in a calcareous and unoxidized condition, then it must have been blown from the Iowan drift before the drift was weathered. This thesis may now be tested by noting other evidences for the age of the loess. 3. The Age of the Loess.-The age of the loess can be as-certained by noting its relations to the Kansan drift, the Illi-noian drift, the Iowan drift and the ·Wisconsin drift. In the ·Kansan area the loess mantles the slopes as well as the uplands. Calcareous and fossiliferous loess in many places also overlies leached, oxidized, and cfocomposed Kansan drift. Such evi-dences of an unconformity tell us that the Kansan drift was not only weathered before the loess ·was deposited, but that the drift-plain was quite thoroughly dissected. The loess mantle continues into the Illinoian drift area in southeastern Iowa, where it shows the same amount of weather-ing as the loess about the Iowan border. Here it also occurs on the eroded surface of the Illinoian and in many places its calcareous zone rests on the weathered and decomposed Illi-noian till, there being in many places an imcrvening develop-ment (}f the gumbo as in the Kansan area except that it is thin-4Proceedings of the Iowa Academy of Science, Vol. 24 [1917], No. 1, Art. 15https://scholarworks.uni.edu/pias/vol24/iss1/15THE IOWAN GLACIATION 91 ner. Thus it is clear that the loess also is younger than the Illinoian drift by a considerable interval. In the Iowan ·area the loess is generally so thin that the leached zone extends down through the loess into the till, but it is a striking fact that in those areas where the loess averages four to six feet in thickness the leached zone passes into the underlying till scarcely more than a few inches to one and one-half feet. Evidently the leaching process has but recently reached the till. This view is supported also by the fact that the top of the till is practically the same color as the lower part of the loess. No soil or gumbo development was found any-where between the Iowan drift and the loess. Where the loess is absent or nearly so the drift is leached correspondingly more, but a little less than the loess where thick sections are exposed. All of these evidences bear out the interpretations made from the areal relations of the loess, its composition and character-istics, that the loess was deposited closely following the retreat of the Iowan ice-sheet. This conclusion may at first seem incompatible with the evi-dence of the fossil shells which, according to Shimek, indicate much the same climatic conditions as the region possesses today. But it should be recognized that the climate at the close of a glacial epoch must be decidedly different from that at the be-ginning. A glacier is the product of glaeial conditions. Its development and advance are ipreceded first by the culmination of glacial temperatures and precipitations. Its retreat occurs only after the glacial climate ceases. Probably the climate of the zone in close proximity to a retreating continental ice-sheet is affected somewhat by the presence of the ice, but yet the conditions are probably much less severe than in the case of an advancing ice-sheet. In the first case, the climate opposes the existence of the ice mass, while in the second, it is respon-sible for it. Is not the close of any glacial epoch in reality that time when the climate becomes permanently effective in causing glacial retreat~ Granted that present temperatures probably did not prevail in the immediate vicinity of the Iowan ice-edge, it does seem likely that after the earth's climate had changed and the ice had melted baek several hundred miles from its extreme limit, seasons apiproximating those of the present prevailed where the marginal loess occurs. Even though this 5Leighton: The Iowan Glaciation and the So-Called Iowan Loess DepositsPublished by UNI ScholarWorks, 191792 IOWA ACADEMY OF SCIENCE VOL. XXIV, 1917 distance be increased to a thousand miles, the length of time con-sumed in this retreat would be brief geologically, probably not permitting of any perceptible amount of weathering. The state-ment, therefore, seems safe that the loess was deposited imme-diately after the closing stages of the Iowan glacial epoch in which .case the locss should be regarded as early Peorian in age rather than Iowan. If the locss is early Peorian in age, the relations of the loess to the \Visconsin drift should show it. In an excellent ex-posure made by the Chicago, Milwaukee & St. Paul Railway Company during the recent reconstruction of their line in Mar-shall county, at the Wisconsin margin, tangible evidence was found supporting this view. Herc the loess, \vhich is typically de-veloped to the south of the Iowan border, passes beneath twenty-five feet of Wisconsin drift. The locss itself also is twenty-five feet thick. The larger part of the Wisconsin drift has been scarcely changed by weathering, whereas the loess is al-most wholly oxidized, there being some gray left at the base, and the top of the loess in one 1Jlace is leached four to five feet. Therefore, the conclusion seems warranted that the loess which is associated with the Iowan drift is chiefly early Peorian in age. 4. Bearing of the Locss on the Problem of the Iowan Drift. -Inasmuch as the deposition of the locss dates back to the time when the Iowan drift was yet unleaehed, the locss may be used for correlation purposes in determining the time relations of the Iowan drift to the Kansan and Illinoian drifts. The great un-conformities between the loess and the Kansan drift, and between the loess and the Illinoian drift, which have already been de-scribed, show that the Iowan ice-sheet invaded Iowa a long time after the Kansan and lllinoian glaciations, the Kansan, of . course, having much preceded the Illinoian. DEPARTMENT OF GEOLOGY, UNIVERSITY OF WASHINGTON. 6Proceedings of the Iowa Academy of Science, Vol. 24 [1917], No. 1, Art. 15ttps://scholarworks.uni.edu/pias/vol24/iss1/15

The Iowan Glaciation and the So-Called Iowan Loess Deposits

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The Iowan Glaciation and the So-Called Iowan Loess Deposits

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