The history of the Champlain Sea is divided into an early cold period of subarctic water termed the Hiatella phase that lasted from its inception, about 11 800 years ago (Two Creeks time), until a low water phase about 10 800 years ago (Younger Dryas or Valders time). This was followed by an interval of warmer boreal water, here called the Mya arenaria phase, that lasted from about 10 800 years ago until 10 200 years ago or the end of the Champlain Sea.General descriptions, previously unpublished, of Groningen dates GrN 2031 (10 870 ± 100 B.P.), GrN 2032(10 450 ± 80 B.P.), GrN 2034 (10 590 ± 100 B.P.), and GrN 2035 (10 330 ± 100 B.P.) are presented. These dates record the Mya arenaria phase of the Champlain Sea, which may have had an initial rising (transgressive) phase, though the stratigraphic evidence, supported by three sets of radiocarbon dates, is not wholly conclusive.