CaO‐Zro2 Molar Ratios as an Index of Weathering

Content of calcium, zirconium, iron, and potassium were determined by X‐ray spectrographic analysis in six loessial soils comprising a chronosequence in southern Illinois. The molar ratio of CaO to ZrO 2 in the fine‐ and coarsesilt fractions yields a negative straight line relationship with the log of the distance from loess source. Relative weathering between the end members of the sequence as expressed both by calcium loss data and CaO/ZrO 2 ratios indicates that major loess deposition ended from 11,000 to 11,800 years ago, a date in good agreement with radiocarbon data for withdrawal of glacial ice from northern Illinois. Using these data, the ratio of time during to time since loess deposition is about 1:1. The data also indicate that the first one‐half to two‐thirds of Peorian loess was deposited faster than the surficial portion.