The occurrence of marine clays and silts in the Chalk River area necessitates a revision of the previously accepted position of the northwestern extent of the Champlain Sea in the Ottawa Valley. The marine origin of these deposits is demonstrated by sedimentological, geochemical, and paleontological criteria. Boron and vanadium concentrations indicate a salinity for this part of the Champlain Sea of from 12 to 16 parts per thousand. Foraminifera present in the clays suggest a shallow brackish water environment. An evaluation of elevations of the marine limit indicates that the sea was present at Chalk River between about 11 300 and 11 100 years BP and thus was a relatively late phase of the Champlain Sea. It appears that ice cover in the area had prevented an earlier inundation by Champlain Sea waters.Till overlying the marine sediments is attributed to a minor readvance starting about 11 000 years ago. The timing and geographic location of this advance strongly indicate a correlation with the St. Narcisse event, well documented to the east of the Ottawa Valley. With the subsequent ice retreat, aeolian and lacustrine and, later, fluvial conditions prevailed, as isostatic recovery had elevated the area above the existing sea level.