Recession of the Laurentide Ice Sheet from northern New Hampshire was interrupted by the Littleton-Bethlehem (L-B) readvance and deposition of the extensive White Mountain Moraine System (WMMS). Our mapping of this moraine belt and related glacial lake sequence has refined the deglaciation history of the region. The age of the western part of the WMMS is constrained to ~14.0–13.8 cal ka BP by glacial Lake Hitchcock varves that occur beneath and above L-B readvance till and were matched to a revised calibration of the North American Varve Chronology presented here. Using this age for when boulders were deposited on the moraines has enabled calibration of regional cosmogenic-nuclide production rates to improve the precision of exposure dating in New England. The L-B readvance coincided with the Older Dryas (OD) cooling documented by workers in Europe and the equivalent GI-1d cooling event in the Greenland Ice Core Chronology 2005 (GICC05) time scale. The readvance and associated moraines provide the first well-documented and dated evidence of the OD event in the northeastern United States. Our lake sediment cores show that the Younger Dryas cooling was likewise prominent in the White Mountains, thus extending the record of this event westward from Maine and Maritime Canada.

The mode of deglaciation in the northwestern White Mountains of New Hampshire has been controversial since the mid 1800's. Early workers believed that active ice deposited the Bethlehem Moraine complex in the Ammonoosuc River basin during recession of the last ice sheet. In the 1930's this deglaciation model was replaced by the concept of widespread simultaneous stagnation and downwastage of Late Wisconsinan ice. The present authors reexamined the Bethlehem Moraine complex and support the original interpretation of a series of moraines deposited by active ice. We found other moraine clusters of similar age to the northeast in the Johns River and Israel River basins. Ice-marginal deposits that probably correlate with the Bethlehem Moraine also occur west of Littleton. The Bethlehem Moraine complex and equivalent deposits in adjacent areas were formed by readvance and oscillatory retreat of the Connecticut Valley lobe of the Laurentide Ice Sheet. This event is called the Littleton-Bethlehem Readvance. Throughout the study area, sequences of glaciolacustrine deposits and meltwater drainage channels indicate progressive northward recession of the glacier margin. Radiocarbon dates from nearby New England and Québec suggest that the ice sheet withdrew from this part of the White Mountains between about 12 500 and 12 000 14 C yr BP. We attribute the Littleton- Bethlehem Readvance to a brief climatic cooling during Older Dyas time, close to 12,000 BP.Le mode de déglaciation du nord-ouest des White Mountains a fait l'objet de contro- verses depuis le milieu du XIX e siècle. Les premiers chercheurs ont supposé que le complexe morainique de Bethlehem dans le bassin de l'Ammonoosuc River avait été mis en place par la glace active au cours du recul du dernier Inlandsis. Au cours des années 1930, ce modèle de déglaciation a été remplacé par le concept selon lequel il y aurait eu stagnation simultanée sur une grande étendue et fonte de l'inlandsis du Wisconsinien supérieur. Les présents auteurs ont réétudié la Moraine de Bethlehem et appuient l'interprétation originelle selon laquelle il s'agit d'un ensemble de moraines terminales mises en place par une glace active. Ils ont aussi découvert d'autres ensembles morainiques d'âge similaire au nord-est, dans les bassins de Johns River et Israel River. Quelques dépôts de marge glaciaire, probablement de la même époque que celle du complexe morainique de Bethlehem, ont aussi été identifiés à l'ouest de Littleton. Le complexe morainique de Bethlehem et les dépôts équivalents dans les zones adjacentes ont été mis en place par récurrence et retrait oscillatoire du lobe de la vallée du Connecticut appartenant à l'Inlandsis laurentidien. Il s'agit de la récurrence de Littleton-Bethlehem. Les dates obtenues en Nouvelle-Angletterre et dans le Québec limitrophe laissent supposer que l'Inlandsis laurentidien s'est retiré de cette partie des White Mountains vers 12 500-12 000 14 C BP. Les auteurs attribuent la récurrence à une refroidissement climatiqu...

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