Abstract: Radiocarbon dates indicate that the maximum time of weathering for soils of the Tazewell substage may be about 6400 years longer than that available for weathering of soils of the Cary substage.
Detailed mineralogical and chemical studies were made of the 2-0.2µ fractions of 0–6 in. and 30–36 in. samples from 24 sites along a traverse extending from the Cary substage across the Tazewell substage in east-central Indiana. The major clay mineral constituents of the 0–6 in. samples were illite and 14 Å material ; illite and montmorillonite were the main components in the 30–36 in. samples. The correlation coefficients between illite content and percentage K2O for the 0–6 in. and 30–36 in. samples were 0.85 and 0.92, respectively.
The end and terminal moraines of the Tazewell substage were found to have a higher illite content than the ground moraines. This was attributed te superglacial movement and enrichment of foreign materials, i.e. mica schists from the Canadian shield, on end and terminal moraines.
The results of saturation of the clay fraction with potassium and magnesium on the proportion of 10 Å, 12.7 Å and 14 Å spacings indicated that weathering had lowered the surface charge density of the micaceous minerals in the 0–6 in. samples to a greater extent than in the 30–36 in. samples. This effect, in addition to other weathering reactions, resulted in partially expanded micaceous minerals in the 0–6 in. samples which were much more resistant to collapse than were the minerals in the 30–36 in. samples.
The rate of weathering of K2O from the 2-0.2µ fraction of the 0–6 in. samples located on ground moraines was estimated to be about 0.1 percent per 1000 years. This value was used in estimating the ages of the morainic systems of the Tazewell in Indiana. The estimated ages were as follows: Mississinewa, (radiocarbon date) 13,140 years; Union City, 14,500 years; Bloomington, 15,500 years; Champaign, 18,700 years; Shelbyville, (radiocarbon date) 19,500 years.
The fertility status of the soils on the Tazewell has been influenced appreciably by the length of the weathering period, the potassium-supplying power of the soils decreasing with increasing age.