Four to five months each year, ice is an active agent of erosion, transportation, sedimentation, and protection in tidal-flat environments along the St. Lawrence Estuary. Characteristic erosional and sedimentary features include chaotic microrelief up to 60 cm high, circular depressions 20 to 50 cm deep and up to a few meters in diameter, furrows 20 to 35 cm deep and up to 2 km long, ice-push ridges, deformational structures, ice-rafted boulders, and clumps of coarse and fine-grained material scattered throughout tidal flats. Every year, millions of tons of sediment of various texture are incorporated into ice, removed from the shore and nearshore zones, and transported over various distances ranging from a few decimeters to many kilometers. During the winter, the ice cov r protects the tidal flats from wave and current erosion and allows deposition of 20 to 35 cm of soft mud under the ice sheet in the macrotidal zone. Modern cold region tidal flats show characteristic sedimentary features, structures, and sequences that can be useful in identifying ancient shelf sedimentary environments.

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