The Formation of Lime Concretions in the Moody and Crofton Series

An area of soils which have been classified in Federal and State soil surveys with the Moody and Crofton series occupies about five thousand square miles in northeast Nebraska and contiguous parts of Iowa and South Dakota. The relief in this area ranges from nearly level to hilly, being characterized at most places by round-topped hills and divides which rise thirty to fifty feet above the intervening valleys. Surface and internal drainage are everywhere adequate. In the more hilly areas, occupied principally by the Crofton soils, the runoff is rapid and erosion is severe. The climate of the area is continental and temperate with wide seasonal variations. The spring season is cool with considerable rainy weather, the summer is long and warm, and the autumn ,is moderate in both temperature and precipitation. Low temperatures occur rather frequently during the winter months but are usually accompanied by snow, which prevents deep subsoil freezing. Fully 75 per cent of the mean annual precipitation, about twenty-eight inches, falls during the principal part of the growing season, April to September inclusive. The average temperature during the coldest months, December, January, and February, is about 25°F. and. during the summer season, June to August inclusive, is approximately 75°F. The Moody series, which includes soils that have developed under grass vegetation from unusually limey loess deposits in the northern chernozem province, have very 1 dark grayish-brown to black topsoils ranging in thickness from seven to twelve inches. The dark surface layers merge downward through a granular dark grayish-brown transition zone, four to six inches thick, into a yellowish-brown, friable and silty subsoil of cloddy structure. A well-developed lime zone lies between the average depths of eighteen and thirty inches. Its upper portion is characterized by an abundance of small, fairly hard, and subspherical lime concretions, oneeighth to one fourth inch in diameter. In the lower portion the concretions are larger, more ellipsoidal, and much less numerous. The parent loess lies at a depth of two to four feet. It contains only an occasional lime concretion, and has no zone of unusual carbonate accumulation. The Crofton soils are on steeply sloping and hilly uplands, where erosion has removed the dark topsoil from the Moody, and the concretionary zone is on or near the surface of the ground. The principal feature of the soils classed with these two series is the band of small rounded lime concretions in their profiles. This paper deals with a description of these concretions, their distribution in the profile, and their probable mode of formation.