Abstract: Types of clay minerals have been determined in samples from three profiles in Indiana: (1) a residual soil from limestone; (2) a Pleistocene till of Illinoian age; (3) a Pleistocene till of Wisconsin age. The type of clay minerals varies with depth from the surface because of degree of weathering. In the residual soil from limestone, montmorillonite is the dominant clay mineral; the entire profile is highly weathered. Likewise montmorillonite is dominant in the highly weathered portion of both the Illinoian till and the Wisconsin till. The relatively unweathered portion of both tills contains illite and chlorite as dominant clay minerals. The factors of the weathering process in central and southern Indiana evidently favor the development of the clay minerals having expanding lattices. The mechanism for changing chlorite and illite to montmorillonite seems to include oxidation of the iron in the lattice and subsequent leaching of magnesium and potassium.
When surface clays are eroded and transported by streams into a marine environment, the expanding-lattice clay minerals may revert readily to the original type, either chlorite or illite.