Reconnaissance along the northern flank of the Appalachian Highland has revealed a new “highland front” morainic system that extends a distance of about 225 miles between Rivière du Loup and Granby, Quebec. This system is thought to have controlled invasion of the St. Lawrence Lowland by the Champlain Sea and, probably at about the same time, the late stages of glacial lakes in the Lake Champlain basin. A new radiocarbon date on shells from Kingsey Falls, Quebec, gives a minimum age of 11,410 ± 150 years for the Champlain Sea, herein redefined, and for the so-called “highland front” moraine. A presumably older morainic system at higher elevation is shown over a distance of about 60 miles between St. Philémon (Dorchester) and Leeds Village, Quebec. Two drainage channels parallel to and south of these moraines are shown; the more easterly, now occupied by the Daaquam River, may have connected the Chaudière Valley to the St. John Valley and may have provided drainage to the Atlantic along the trend of the Appalachians at a time earlier than about 12,720 years B.P.