The pre-Woodfordian drift of north-central Wisconsin is composed of at least two units, the Merrill and Wausau tills, which are distinguished from each other and adjacent Woodfordian deposits on the basis of pebble lithology, ice-flow direction, and semi-quantitative clay mineralogy. The Merrill drift is stratigraphically above the Wausau drift, and the lower unit has a patchy distribution and is commonly absent in the area covered by the Merrill till. The degree of weathering of the clay fraction (decrease in illite, chlorite; increase in vermiculite, montmorillonite) of the Merrill till indicates that it underwent a period of subaerial weathering before deposition of the relatively unweathered Woodfordian age drift to the north. A radiocarbon date suggests the Merrill till is > 40,800 years old (IGS-256). The Wausau till shows an even greater degree of weathering than the Merrill till and represents an earlier, separate ice advance. Both tills show evidence of in situ weathering of the clay minerals.