ABSOLUTE CHRONOLOGY OF ICE-FLOW EVENTS IN NORTHERN NEW ENGLAND: A WORKING HYPOTHESIS AND PROCESSES IMPLICATIONS

Mapping of glacial striations in northern New England produced a relative sequence of events (oldest flow toward the E and in extreme northern New England a flow reversal to NW and then N whereas elsewhere the E flow shifted to SE). Their placement into a numerical chronology depends on bracketing radiocarbon ages and correlations. Several sites in northern New England contain relic soils beneath lacustrine sediments (e.g. Isie Lake, 24,300±110, OS-6435; Jo Mary Pond, 24,500±130 OS-3170) which bracket the beginning of extensive glacier cover. Numerous AMS basal ages on terrestrial macrofossils require deglaciation of northern New England before 10,400 14C yr BP. Correlations to the marine records and other terrestrial records within this 14,000 yr. time slice indicate that ice rafted debris (IRD) concentrations grow and peak at the same time as advances along the extreme southern margin of the Laurentide Ice Sheet, suggesting at least four general expansions and contractions of the ice masses.

For discussion we make the following assignments: Strong eastward flow from 24,000 to 20,800 14C BP; general decreasing of flow strength and beginning of reversal from 20,800 to 16,000 14C BP with increased strength of northward flow until 14,000 14C BP; shift to NW flow about 12,500 14C BP as calving bay migrated up the St. Lawrence Seaway; localized activity until 10,400 14C BP.

These correlations imply that the erosion periods are relatively short (~3,000 yr), - but in light of the minor total erosion it appears that actual erosion duration is even shorter. Another implication is that the first sheet flow (E) did not simply drain down the topography of the St. Lawrence lowland but later yielded gradually to streaming flow. It would seem that ice streams develop and migrate during times of low or negative mass balance over a few thousand years interval.