The distribution and age of glaciomarine and marine sediment in the northern Puget Lowland, Washington, demonstrate that rapid retreat of continental ice, the Everson marine incursion, and high rates of isostatic rebound occurred between about 13 600 and 11 300 14 C yr B.P. (11.3 ka). Glaciomarine and marine deposits are thickest in zones where retreating ice lobes grounded, in the northeast Puget Lowland, and near large drainages. Glaciomarine sediment was deposited mainly from (1) submarine outwash in ice-proximal zones; (2) turbid underflows, dispersed melt water, icebergs, and resedimentation in transitional zones; and (3) dispersed melt water and currents in icedistal zones. Marine, estuarine, and emergence (intertidal and beach) facies accumulated in areas more than 10 km from ice margins, particularly near major rivers. Molluscan and foraminiferal assemblages in the glaciomarine and marine deposits indicate that turbid, cool, brackish water covered much of the Puget Lowland during the Everson interval. Water was generally shallower (<30 m) in the southern part of the area and deeper (15-60 m) to the north. Mineralogy and geochemical properties such as boron or sodium content of the gravel-free fraction do not clearly distinguish glaciomarine and marine deposits from terrestrial deposits. Isostatic rebound rapidly lifted the glaciomarine and marine deposits through sea level between about 13.5 and 11.3 ka. The present altitudes of radiocarbon-dated shell and the marine limit show that initial rates of isostatic rebound exceeded 10 cm yr ؊1 in the northern Puget Lowland, but dropped to 2 cm yr ؊1 before 11 ka. The uplift gradient is about 0.6 m km ؊1 to the north and steepens locally to at least 1.3 m km ؊1. The pattern of emergence in the northern Puget Lowland is anomalous locally, perhaps as a result of complex isostatic effects near the glacier margin, rapid rise of sea level, or tectonic deformation.
Although two physically distinct tills of different ages have long been recognized in southern New England, only in the past decade or so has the existence of a similar two-till stratigraphy been recognized in New Hampshire. In southern New England, distinction between the two tills, a lower one and an upper one, initially was based on differences in texture and weathering: the lower compact till has a siltier matrix and an oxidation zone of 10 or more meters; the upper till has a much sandier matrix and an oxidation zone only in the uppermost 1 m. Later, identification of structural features at the contact between the two tills helped to distinguish them. The same stratigraphic relationships are now recognized in all parts of New Hampshire.Excellent exposures at Nash Stream in northern New Hampshire have provided the most complete inland till stratigraphy to date in New England. The tills exposed here are called the Nash Stream (lower) Till and Stratford Mountain (upper) Till; associated deglacial outwash has also been recognized for each of them. The Nash Stream Till is nonoxidized where it is covered by its associated outwash and is oxidized to a depth of 6 to 7 m where it is exposed at the surface. This suggests that a significantly long weathering interval took place before the last ice sheet deposited the Stratford Mountain Till and its associated outwash.The Nash Stream Till, here correlated with the lower till of southern and central New England, may be an early Wisconsinan correlative of the New Sharon Till in Maine and perhaps of the Johnville Till in southeastern Quebec and the Becancour Till in the St. Lawrence Lowland, or it may be even older. The Stratford Mountain Till, of late Wisconsinan Age, is correlated with the surface till throughout New England, and with the Lennoxville Till in Quebec and with at least the upper part of the Gentilly Till in the St. Lawrence Lowland. No middle Wisconsinan units have so far been recognized in New Hampshire and southern New England.
The mode of ice retreat after the maximum advance of the Wisconsinan glacier that last covered New England has been a subject of controversy for more than 100 years. Two major opposing views dur ing most of this period focused on whether recession was characterized by systematic retreat of active glacier ice or by regional stagnation. Difficulty in correlating with the well-established ice-recessional history in the Middle West hampered the discussion in New England. In the last few decades, detailed mapping on large-scale topographic maps has formed the basis for a third model of deglaciation, the morphosequence concept, which contains parts of both previous views. Careful outlining of the distribution and age relationship of melt-water deposits shows that the ice sheet receded by a process of stagnationzone retreat and that the region was deglaciated systematically. End moraines and readvance localities that demonstrate the presence of live ice during retreat in New England are relatively scarce; however, the distribution of such localities indicates that live ice was only a few kilometers from the margin throughout recession. The position, volumes, and especially the altitudes of melt-water deposits suggest that their source material was debris at or near the ice surface. The debris was carried upward from englacial positions to the ice surface along shear planes that resulted from live ice moving over the obstructing stagnant ice at the glacier margin. Analogous shear planes carrying debris have been found in modern valley glaciers.
A question is posed regarding the source of melt-water sediment. Does stagnant ice, functionally separated from active ice and gradually melting in place, contain enough rock debris to account for the volume of melt-water deposits known to exist in deglaciated areas, or does the volume of these deposits require a sediment source in close association with active ice from which the supply of rock debris is continually replenished?Differing opinions on this question are implied in two contrasting models for deglaciation in the north-eastern United States. One, involving regional stagnation, assumes a sediment source in stagnant glacier ice; the other, involving stagnation-zone retreat, considers active ice the principal sediment source. This paper presents reported values of debris content in glacier ice and uses these values to calculate theoretical sediment volumes for a small drainage basin (1 300 km2) in northeastern U.S.A.A typical value for the amount of rock debris in temperate glacier ice is 25% (volume) debris content in a 400 mm-thick basal debrisrich zone. This value gives a calculated sediment volume of 0.13 km3, about 6% of the estimated actual volume of melt-Hater sediment in the test basin. Comparison of calculated theoretical sediment volume with the estimated actual sediment volume in the basin indicates that stagnant ice is an inadequate sediment source, and that active ice, rather than stagnant ice, is probably the principal sediment source for melt-water deposits.
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