Department of Earth and Environmental Sciences, University of Waterloo
Marl is a soft, light-coloured (white to pale gray or) mud-like sediment, seen in shallow waters of some small lakes or ponds, or under swamp lands, where it typically is covered by black organic sediment (Figure 1).
It usually contains an abundance of visible mollusc shells (snails, clams) as well as a host of other fossils best seen under a microscope (ostracodes, diatoms, insects). The term marl has been applied to a variety of sediments, including marine sediments, and calcareous till or varved clay because they effervesce (“fizz”) vigorously when a drop of acid is applied. However, today the term is normally limited to freshwater sediment.