QUANTITATIVE DATA ON COARSE DEBRIS OCCURR ING ON THE ICE COVER, ST-LAWRENCE ESTUARY

At several sites along the shores Of the St. Lawrence Estuary, the ice cover is laden with coarse debris. A survey of large clasts occurring at the ice surface has been made in four sites. Over 300 clasts of va rious lithologies weighing one kilogram to one metric ton have been me asured. In the four study sites small rock fragments are more abundant ; 60% belong to the 1 to 20 kg category, while 25% are found in the 20 to 50 kg category and the 15% remaining include boulders from 50 kg t o one metric ton. In three sites studied the breakup occurred less tha n 24 hrs after the surveys. Rock fragments at the ice surface have thu s been transported offshore, possibly up to 30 km downstream. Wave ene rgy and current velocity in tidal zones along the St. Lawrence Estuary are too slow to be able to move clasts over 10 cm. Drift ice is the o nly agent capable of transporting these clasts. Such a survey of rock fragments on the ice surface is a useful tool to evaluate adequately t he load of the ice cover and to quantify the role of ice as a erosion and transportation agent.