Phase 3 of the Champlain Sea is defined as the interval that followed the retreat of the Laurentide ice from the Saint-Narcisse Moraine. Events included are the retreat of ice from major valleys in the Laurentian Highlands, invasion of these by the Champlain Sea and regression of the Champlain Sea from its entire basin.
In the lower Saint-maurice Valley, post-Saint-Narcisse retreat of an active ice-front is recorded by the Cossetteville parallel ridges. Transverse ridges of the middle Saint-Maurice Valley and Lac du Missionnaire indicate the presence of an active ice-front in these valleys. Elsewhere, ice-frontal constructions and raised outwash trains were formed at the mouth of valleys, e.g. the proglacial delta perched at an altitude of 200m, at the mouth of Lac Mekinac.
The Champlain Sea extended into the Laurentian Highlands in the form of brackish or freshwater bodies considered as paramarine basins (Occhietti, 1977)
At the close of Phase #, considerable shoaling had already taken place and waters in the Champlain Sea basin turned fresh upstream of Quebec. (Elson, 1969) called this freshwater body Lake Lampsilis, "after a freshwater clam commonly found as a fossil near it's shores" (Elson,1969). Development of Lake Lampsilis in the basin was in conjunction with the time-transgressive retreat of the saltwater wedge. Surface waters had turned fresh in the lower reaches of the Ottawa Valley by about 10.3ka . Marine waters however continued to penetrate into the fresh waters of Lake Lampsilis until very late in the Quebec area, presumably because of strong tides. Retreat of the saltwater wedge, which marked the end of the Champlain Sea Phase 3, was diachronic at the scale of the basin; its age ranges from about 10ka in the Ottawa Valley to about 9.3ka in the vicinity of Quebec.