Deglaciation of the northwestern White Mountains, New Hampshire

Abstract

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...

Varve, paleomagnetic, and 14 C Chronologies for late Pleistocene events in New Hampshire and Vermont (U.S.A.)

“…With additional fieldwork in the area he extended the Bethlehem Moraines to 7 km west of Littleton to include ice-marginal deposits in the Mullikin Brook valley. Careful reexamination of the sections on Mill Brook have revealed a more complete stratigraphy than seen by Crosby (Ridge et al, 1996;Thompson et al, 1999). On a higher bank about 250 m up stream from Crosby's section is an exposure of three till units.…”

“…Prior to 1910 morainic topography from Bethlehem to Littleton, New Hampshire was interpreted to be the result of northward flowing valley glaciers from the White Mountains at the end of the last glaciation (Agassiz, 1870;Hitchcock, 1878;Upham, 1904; see Thompson et al, 1996Thompson et al, , 1999Thompson, 1999). James W. Goldthwait (1916) re-interpreted the Bethlehem Moraines as ice-marginal deposits built at the southern margin of a receding continental ice sheet.…”

“…5) over 50 m of till and preglacial gravel sit on bedrock and have been downcut by the modern Connecticut River that today runs on bedrock. Till on both sides of the valley is capped by varves deposited during deglaciation (Ridge et al, 1996;Thompson et al, 1999). During deglaciation the valley was filled with till and preglacial gravel and could not have been a location where a deep varve section accumulated significantly below the elevation of existing exposures along the river.…”

“…8). The exceedingly thick varves occur at about the time that lakes in the upper Ammonoosuc Valley east of Littleton (Lougee, 1940;Thompson et al, 1996Thompson et al, , 1999 catastrophically drained to the Connecticut Valley in response to the recession of ice from the Bethlehem Moraines. The Ammonoosuc Valley floods may represent the largest release of lake water impounded in a tributary during the recession of ice in the upper Connecticut Valley.…”

Applying the Cosmogenic Nuclide Dipstick Model for Deglaciation of Mt. Washington

“…Perhaps a local Appalachian ice sheet or ice cap covered the mountainous areas of northern New England during parts of the late Wisconsinan, an idea invoked long ago by Flint (1951). However, the moraine record in the lowlands adjacent to the Presidential Range to the north and the North American varve record from glacial Lake Hitchcock to the west suggest continental ice recession toward the north during deglaciation (Thompson et al, 1999(Thompson et al, , 2017Ridge;Ridge et al, 2012;Bromley et al, 2015). Bierman et al (2015).…”