A succession of molluscan faunas preserved in the upper 366 cm of a vibracore recovered from the Cowles Bog area records a series of long- and short-term environmental changes in the Lake Michigan basin since the low-water Lake Chippewa phase.

The rise in the water to the Nipissing I level during the post-Lake Chippewa transgression affected the water table landward from the lake and brought into existence a small lake or pond at the core site. This event is recorded in the core at a depth of about 366 cm by a change in lithology from interbedded organic layers and fine sand to a fossiliferous, calcareous micrite (marl) and by the appearance of molluscs, in which the overwhelming preponderance of taxa and individuals prefer unpolluted, well-oxygenated perennial aquatic habitats. This molluscan assemblage continues to dominate the core between 366 cm and 190 cm. Between 182 cm and 122 cm, this association of species is almost completely replaced by taxa tolerant of water bodies that are becoming filled with organic debris, by species that can inhabit temporary water bodies, and by marsh-inhabiting terrestrial species. Paludification of the site, from 122 cm to the surface, is indicated by an assemblage dominated by terrestrial species that prefer wet to very moist substrates. These occur in association with pisidiid clams and aquatic gastropods capable of living in seasonal bodies of water.

Short-term climatic events are suggested by rapid increases in the abundance of aquatic taxa in the upper 100 cm of the core. These faunal changes are interpreted as responses to a rise in the local water table due to increased precipitation.