Gumbotil—Its Formation and Relation to Overlying Soils with Claypan Subsoils

G deposits of Kansan agecover practically all of northern Missouri. On the broad, level interstream areas, representing the original drift plain, the upper part of the till is in places a dense, plastic gray clay, known as Gumbotil. The clay is everywhere overlain and preserved by a deposit of loess, probably of Peorian age, and 4 to 6 feet in thickness. The modern soil formed from the loess is characterized by a claypan subsoil. The purpose of this paper is to give an explanation of the origin of the gumbotil, and to show its possible relation to the formation of the claypan in the loess-drived soil above. Gumbotil is the name applied by Kay to the dense gray clay that forms the upper part of the till on the' broad level uplands. The clay is gummy, tenacious, nonlaminated, and remarkably uniform in color and texture. The color is light gray. It is estimated that more than 50% of the clay is colloidal in size. It is sticky when wet and hard when dry. There are no pebbles or evidence of decomposed rocks, but occasional small bits of chert occur. Small iron concretions occur throughout the mass. The clay varies in thickness from i foot to more than 12 feet, but in most places the range is between 4 to 6 feet. Extremes in thickness may occur within a distance of several hundred feet. Under these conditions it generally occurs as a broad V-shaped mass. Observations thus far seem to indicate that gumbotil is usually thicker and more extensive in regions of shallow till (less than 20 feet) than in regions of deep deposits. All of the characteristics of the gumbotil clearly indicate that it is a water deposit, or lacustrine in origin. Gumbotil is always a surface formation, and is clearly differentiated from the yellow till upon which it rests. It was deposited as a gumbotil and did not become so subsequent to its deposition. It was weathered and leached when deposited. The gray bleached color and the segregated iron further indicate that the clay existed under saturated or submerged conditions for a long period of time. The fine texture and the uniformity of the clay also clearly indicate aqueous origin. The' origin of the gumbotil has been interpreted by geologists and by some pedologists as a product of long-time weathering. It is defined as a highly weathered till, in which all coarse material has been decomposed into clay. It is the present residium of the whole chemical leaching process. Such an interpretation of the origin of gumbotil is incompatible with present-day concepts of podogenic processes. It is inconceivable that such extreme weathering as characterizes the gumbotil could extend to great thickness of 10 feet or more. Nowhere are thick weathered clay strata known to occur in temperature regions, even on the oldest land surfaces. Pedogenic processes do not extend to such depth, and with such uniformity. If the gumbotil is a weathering product, then there should be present quartz sand and other resistant minerals. Gumbotil is characterized by the great uniformity of the clay mass. The sharp contrast and contact between the gumbotil and the slightly weathered and sometimes unleached till below could not possibly result from weathering 'processes. Furthermore, the great variation in thickness and the irregular distribution of the gumbotil cannot be explained by weathering. In brief, the interpretation of gumbotil as a weathering product of till in place is not applicable to conditions in Missouri. Recent construction of deep road cuts has made it possible to observe the gumbotil in relation to the underlying and surrounding till, and also to determine the occurrence and shape of the clay deposits. Although it is confined to the higher upland, it does not occur as a continuous horizon or strata on the upland plain. It occurs in depressions, swales, and shallow basins in the original drift plain. These depressions may be several hundred feet to several miles in width. If they could be traced in their entirety, they would be found to form buried valleys, with a general north and south trend. Such depressions and basins are characteristic of young drift plains. They became filled with clay (gumbotil) brought in by slow-moving water from the surface of the surrounding glacial deposits. The water was not from a receding ice front, but from the glacial surface. It was a condition that is occurring even . -today on the level prairie land of northeastern Missouri. The broad shallow depressions are gradually aggrading with a gray silty material that is continually washed from the surrounding land. In the center of the depressions the gumbotil may have a thickness of 10 feet or more, and gradually thins toward the edges. A cross-section has the appearance of a filled-in sink hole. This varied thickness' and occurrence of the gumbotil explain why the clay may outcrop on one side of a ridge and not on the other and why the outcrops may be of different thickness. It should be noted that the surface of the original glacial plain has been modified in most places