The distribution of 50 freshwater marl accumulation sites is compared with terrain attributes. All marl sites are underlain by clay and surrounded by terrain that is dominated by glacial till. Sites grouped in a western marl area appear to have been derived from thick calcareous till, but those in an eastern area are surrounded by thin till on highly jointed and fractured limestone bedrock. Many bottomland marls are in riverine locations, i.e., in the basins of the Collins and Millhaven Creeks and of the Salmon and Moira Rivers. About three quarters of the marl sites contain mantling peat.Sixteen radiocarbon datings disclose that marl formation began and ceased as early as 11 180 and 9330 years BP, respectively. Subsequent termination of marl formation transgressed time into the present: most upland sites stopped marl production prior to 8000 years BP and those that persisted dropped out between 6000 and 4000 years BP. Many bottomland marl sites changed into peat-forming sites between 4000 and 2000 years BP and show an apparent trend of progressive up-basin termination of marl formation. The causes of these trophic changes are speculated on from the soil–geomorphological perspective.