Many new exposures have been made by the Chicago, Milwaukee and St. Paul Railway in the reconstruction of their line across Iowa, Various ones of these have proved to be of especial interest to Pleistocene geologists, and among them are several cuts in the northern part of Clinton County, showing superimposition of the two oldest drifts, the Kansan drift on the Sub- Aftonian drift. This paper is devoted to a description of these and their interpretation

Leighton, Morris M.

English

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 23 | Annual Issue Article 211916Superimposition of Kansan Drift on SubaftonianDrift in Eastern IowaMorris M. LeightonUniversity of WashingtonCopyright © Copyright 1916 by the Iowa Academy of Science, Inc.Follow this and additional works at: https://scholarworks.uni.edu/piasThis 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 inProceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contactscholarworks@uni.edu.Recommended CitationLeighton, Morris M. (1916) "Superimposition of Kansan Drift on Subaftonian Drift in Eastern Iowa," Proceedings of the Iowa Academyof Science, 23(1), 133-139.Available at: https://scholarworks.uni.edu/pias/vol23/iss1/21KANSAN DRIFT ON SUB-AFTONIAN 133SUPERIMPOSITION OF KANSAN DRIFT ON SUB-AFTONIAN DRIFT IN EASTERN IOWA.MORRIS M. LEIGHTON.Many new exposures have been made by the Chicago, Milwaukee and St. Paul Railway in the reconstruction of their lineacross Iowa, Various ones of these have proved to be of especialinterest to Pleistocene geologists, and among them are severalcuts in the northern part of Clinton county, showing superim-position of the two oldest drifts, the Kansan drift on the Sub-Aftonian drift. This paper is devoted to a description of theseand their interpretation.1. A significant exposure is located at the second viaduct one-half mile east of Delmar Junction. The cut is through a dividewith a rounded summit, 250 to 300 yards long, and has a maximum depth of sixty feet. By reference to figure 13. A, therelations of the following materials will be clear :BSub-AftcnunTillFIG. 13 A—Diagrammatic sketch of the relations of the materials shown Inthe south side of the Chicago, Milwaukee and St. Paul railway cut, one-half mile east of Delmar Junction.Fid. 13 B—Cross-section of the railway cut referred to In A, showing theformer track-level, m; the position of the mineralized stump, c; and thepresent track-level, n. 1Leighton: Superimposition of Kansan Drift on Subaftonian Drift in Eastern IPublished by UNI ScholarWorks, 1916134 IOWA ACADEMY OF SCIENCELOESSKANSANDRIFTAFTOXIANSOILSCB-AFTONIANTILLFEET. Loess, 1 ft. of soil at top, grading belowinto brownish yellow to buff loess, whollyleached of calcareous material, mantles theeroded surface of the Kansan drift; thickness at the summit 8-106. Ferretto zone at the top of the till, absentfrom the slopes, reddish brown, leached,pebbles show considerable decomposition;thickness 0-1 ViGrades downward into:0. Till, brownish yellow to yellow, summitrounded, leached of calcareous matrix andlimestone pebbles in uppermost 7 to 8 ft.,calcareous below with lime concretionsand limestone pebbles, insoluble drift pebbles present throughout, lime concretionsmost abundant just below the base of theleached portion; maximum thickness 25Grades downward into:4. Till, blue-gray or slate-colored, containingtwo large sand pockets, a and 6, whichhave the appearance of included bodies,sand pocket a lies in the transition zoneof (4) and (5), matrix of till calcareousand limestone and other drift pebbles present, fragments of wood in the basal portion,Jills an old depression; thickness 0-283. Old black soil, with many small fragments ofwood mineralized with iron pyrite, pebblesrare, some imperfectly laminated clay, soil-zone delineates an old depression withslopes as high as 12°. At c, on an oldtrack level (Fig. 13, B), Is a stump withroots and rootlets running through theold soil and underlying clay; the wood ismineralized like the fragments of woodthroughout the soil zone. Thickness ofsoil zone 2V4-3Grades downward into:2. Ti;l, dark bluish green on damp surface,light grayish green where dry, leachedof calcareous matrix and limestone pebblesbut other drift pebbles are present; thickness 6±Grades downward into:1. Till, yellowish to brownish green, with somemaroon-colored material in the lower part,leached 2-4 ft., calcareous below; thicknessexposed 0-142Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 21https://scholarworks.uni.edu/pias/vol23/iss1/21KANSAN DRIFT ON SUB-AFTONIAN 135Interpretation.—This exposure seems to show quite clearly twodistinct tills. The dark bluish green color of the till below theold soil zone is the color of material which has undergone de-oxidation from a former oxidized state. Its transition belowinto material of yellowish to brownish color, with a greenishtinge, and the presence of carbonaceous material directly above,indicates that it was once oxidized but has been subsequently deoxidized, probably since the soil material has been deprived ofatmospheric oxygen. The leaching of the lower till to a depth ofabout eight feet requires an interval of time, much longer thanpost-Wisconsin time, during which oxidation would likely havetaken place. Directly above the soil zone is till which has neverbeen leached or oxidized. These relations show quite clearly thatthe deposition of the two tills was separated by an intervalworthy of the designation of an interglacial epoch.The upper till-body is unquestionably Kansan. The topography of the surrounding region is not only erosional, like theKansan area, but the upper till is weathered similar to the tillof the Kansan area, both from the standpoint of degree and depthof oxidation and from the standpoint of depth of leaching.Hence, if the upper till is Kansan, the lower till must be sub-Aftonian. and the soil development and leaching and oxidation ofthe lower till must be Aftonian in age.The age of the stump just at the old track-level was carefullyconsidered. Upon examination of various parts of the stumpand its roots, it was found that the wood is mineralized with ironpyrite just like fragments of wood which are scattered throughout the Aftonian soil-zone. This evidence led to the conclusionthat the stump belongs to the Aftonian soil and was uncoveredin the excavation of the cut. This attention has been given tothe stump, not because it necessarily adds to the weight of evidence for the differentiation of the two till-bodies, but it is thefirst stump, having an interglacial position, known to have beenreported.The position of the loess above the ferretto of the Kansandrift and as a mantle on the eroded surface of the Kansan drift,warrants the usual interpretation that a considerable intervalof erosion and weathering intervened between the deposition ofthe Kansan drift and that of the loess about the lowan driftborder. 3Leighton: uperimposition of Kansan Drift on Subaftonian Drift in Eastern IPublished by UNI ScholarWorks, 1916136 IOWA ACADEMY OF SCIENCE: 2:. .Ainotker important cut is just west of the depot at Del-mar Junction, on the north side of the tracks. This cut hasa maximum depth of about twenty feet, is one hundred yardslong and its summit is round. The materials exposed are as follows (figure 14, A) :BFio 14 A—Diagram of relations of materials in the north face of the Chicago, Milwaukee and St. Paul railway cut, just west of the depot atDelmar Junction, Clinton county.Fio 14 B—Diagram of the relations of the materials In the north face of theChicago, Milwaukee and St. Paul railway cut, six miles east of DelmarJunction, Clinton county.LOESSKANSANTILLAFTOXIANSILTSSUB-AFTONIANTILL1.6.5.Sandy loess, buff, well stratified, leached,thin soil zone at the top, conforms to therounded summit of the underlying materials; maximum thickness 10-12Till, brownish yellow to yellow, calcareousmatrix, limestone pebbles present togetherwith other drift pebbles up to the base ofthe loess where there is a concentration ofpebbles; maximum thickness 8±Till, blue-black, containing some wood, lineof oxidation above quite sharp, probablydue to organic material, but calcareouslike the above; thickness 1-34. Silts, dark brown, laminated, appear to havemuch carbonaceous substance, leached;thickness 1-1%Grades downward into:3. Silts, gray, laminated, leached 3f 2. Till, brownish yellow to yellow, with maroonstreaks and some lamination, pebbles andcobbles up to 6 in. in diameter, leached4% ft. In one place, calcareous and limestone pebbles present elsewhere up to thebase of the gray silts, some decayedpebbles; thickness, maximum 8±1. Till, bluish drab, calcareous; thickness 0-34Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 21https://scholarworks.uni.edu/pias/vol23/iss1/21KANSAN DRIFT ON SUB-AFTONIAN 137Pebble counts from the unleaehed portions of both tills yieldedthe following results :KANSANTILLPER CENTSUB-AFTOMANTILLPEB CENTKINDGreenstone and Dolerite 46 42Limestone 30 28Granite . . 8 6Chert 4 8Quartzite 6 6Schist 4 2Quartz 2 2Volcanic Porphyry 0 6100 100Interpretation: —In this exposure, which is one-half mile fromthe former, the existence of the non-calcareous silts, containingcarbonaceous material, between the calcareous till above and theleached and calcareous till below, makes it obvious that herethere are also two till-bodies of different age. Inasmuch as thisgeneral region is one of mature erosion, as mentioned in theformer case, the upper till is believed to be Kansan till, the siltsAftonian, and the lower till sub-Aftonian. The relations ofthe loess to the Kansan till in this cut do not clearly show aninterval between their dates of deposition, but in view of the relations in the cut first described and of the mantling nature ofthe loess, it is probable that the rate of leaching of the Kansantill was at least equaled by erosion and consequently no leachedzone remains. The lithology of the two tills, according to thepebble count, does not show any marked difference.3. Farther east, about six miles east of Delmar Junction andone mile northeast of Riggs, a Chicago, Milwaukee and St. PaulRailway cut, trending north 30° east, fifty to sixty feet deep,and two hundred yards long, exposes two bodies of till, separated by a body of gravel. By referring to figure 14, B, therelations of the materials, which are described below, will beclear.FEET7. Loess, soil-layer at the top 1 ft. thick, brownishyellow and non-calcareous down to the till, snailshells only in the calcareous portion; maximumthickness 30±6. Ferretto zone of till, reddish brown, absent on lowerslopes 0-1% 5Leighton: Superimposition of Kansan Drift on Subaftonian Drift in Eastern IPublished by UNI ScholarWorks, 1916138 IOWA ACADEMY OF SCIENCE5. Till, brownish yellow to yellow, leached 6 feet, calcareous and limestone pebbles below, upperhorizon conforms to the contour of the hill andmantled by the loess, maximum thickness 20Grades downward into:4. Till, gray-blue, calcareous, occurs below thickestpart of the oxidized till and in a small depression of the underlying gravel; maximumthickness 83. Sand and gravel body, extends across the cut exceptwhere mantled on the slopes by loess; limestoneand other drift pebbles present, yellowish tobrownish in color, lens and pocket-structure;thickness 10-122. Till, light drab to dark drab at the top, brownishgray to dark gray below, dense and compact,limestone pebbles to the top, mostly decayed inthe upper one foot, contains some inclusions ofthe underlying silt; thickness 12-141. Silt, somewhat sandy, yet compact, dark gray, nopebbles, fragments of wood or roots of wood 1 to2 inches in diameter exposed near the bottom,upper horizon somewhat undulating and in placesshows gouging by an over-riding ice-sheet;thickness 12±Interpretation: —There is no zone of leaching and oxidationwithin the drift materials which warrants a separation intotwo distinct tills. It has been thought, however, in view ofthe other exposures, that possibly the gravel-body representssuch an interval as the Aftonian and that the overlying drift isKansan in age and the underlying is sub-Aftonian. In thiscase, the bottom silt formation would be probably pre-Pleisto-cene. These determinations, however, must remain somewhatconjectural.It is quite clear, however, that the loess formation was deposited on the Kansan drift after the latter had been eroded andweathered to its present state. This means a relatively longinterval between the deposition of the two as compared with post-Wisconsin time.SUMMARY OF THE CHIEF POINTS.1. These exposures definitely record the invasion of the sub-Aftonian ice-sheet into the extreme eastern part of Iowa. Taking into consideration the other known exposures of sub-Afton6Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 21https://scholarworks.uni.edu/pias/vol23/iss1/21KANSAN DRIFT ON SUB-AFTONIAN 139ian drift in Iowa and Nebraska, it appears that in a broad waythe sub-Aftonian and Kansan ice-sheets covered approximatelythe same territory from east to west, a territory much more extensive than was covered by any of the later ice-sheets in theKeewatin field.2. Where the sub-Aftouian drift is definitely differentiatedfrom the Kansan drift, the sub-Aftonian shows a leached zoneconsiderably deeper than that of the Wisconsin drift. On thisbasis the length of the Aftonian interval was considerably greatert'hflin post- Wisconsin time. This is in harmony with the evidence of the Aftonian mammalian fossils that the interval waslong and warm, and when both evidences are considered it is tobe inferred that the ice-sheet was melted back at least to itspresent limits.3. The existence of the two oldest drifts in this locality andthe absence of any evidence that the Maquoketa and Wapsipini-con river valleys below Monticello and Anamosa, have been occupied by an ice-sheet, indicate that these superimposed valleyshave been carved since the Kansan ice invaded this region.4. The Kansan drift was weathered and eroded to its present state before the loess in this locality was deposited, hencethis weathering records an interval much longer than post-Wisconsin time. The weathering and erosion of the Kansan driftdoes not, therefore, represent its age. The loess itself showsthree to four times the leaching that the Wisconsin drift shows,and the length of time represented must be added to that shownby the weathering and erosion of the Kansan drift, in order tofully appreciate the great age of the Kansfin drift.DEPARTMENT OP GEOLOGY,UNIVERSITY OP WASHINGTON.SEATTLE, WASHINGTON.7Leighton: Superimposition of Kansan Drift on Subaftonian Drift in Eastern IPublished by UNI ScholarWorks, 1916

Superimposition of Kansan Drift on Subaftonian Drift in Eastern Iowa

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Superimposition of Kansan Drift on Subaftonian Drift in Eastern Iowa

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