Mega boulders with a frontal tail on the clayey tidal flats of the lower St.Lawrence estuary

摘要

Mega-boulders with a bouldery tail located offshore are a common feature on the clayey erosional tidal flats of the south shore of the Lower St. Lawrence. Up to 20 in long, the tail is mainly made of coarse debris (cobbles and small to medium size boulders) which overlies a marine clay substrate. The tail is elongated and slighly inclined seaward. This particular deposit in front of isolated mega-boulders (erratics) is not caused by wave diffraction and refraction phenomena. Deposition of stones in front of mega-boulders resulted from shore ice processes and tide currents (ebb). Most features presently observed are subrecent; they were built during low sea-level stages during the Holocene, periods during which erosion surfaces were cut into clay deposits containing abundant dropped stones. Boulder pavements are also common in areas with mega-boulders.