Ernest H. Muller

Tsivat Basin Conduit System persists through two surges, Bering Piedmont Glacier, Alaska

The 1993–1995 surge of Bering Glacier, Alaska, occurred in two distinct phases. Phase 1 of the surge began on the eastern sector in July, 1993 and ended in July, 1994 after a powerful outburst of subglacial meltwater into Tsivat Lake basin on the north side of Weeping Peat Island. Within days, jokulhlaup discharge built a 1.5 km 2 delta of ice blocks (25–30 m) buried in outwash. By late October 1994, discharge temporarily shifted to a vent on Weeping Peat Island, where a second smaller outburst dissected the island and built two new sandar. During phase 2, which began in spring 1995 and ended within five months, continuous discharge issued from several vents along the ice front on Weeping Peat Island before returning to the Tsivat Basin. Surge-related changes include a five- to six-fold increase in meltwater turbidity; the redirection of supercooled water in two ice-contact lakes; and an increase in the rate of glaciolacustrine sedimentation. U.S. Geological Survey aerial photos by Austin Post show large ice blocks in braided channels indicating excessive subglacial discharge in a similar position adjacent to Weeping Peat Island during the 1966–1967 surge. During the subsequent three decades of retreat, the location of ice-marginal, subglacial discharge vents remained aligned on a linear trend that describes the position of a persistent subglacial conduit system. The presence of a major conduit system, possibly stabilized by subglacial bedrock topography, is suggested by (1) high-level subglacial meltwater venting along the northern side of Weeping Peat Island during the 1966–1967 surge, (2) persistent low-level discharge between surges, and (3) the recurrence of localizing meltwater outbursts associated with both phases of the 1993–1995 surge.

Late Wisconsinan, pre-Valley Heads glaciation in the western Mohawk Valley, central New York, and its regional implications

The history of late Wisconsinan, pre- Valley Heads glaciation (about 18-15.5 ka) implies that major glaciological controls, as well as overall climatic warming, influenced the pattern of deglaciation in the western Mohawk Valley. The Middleville Formation in the West Canada Valley contains the most complete record of pre-Valley Heads ice recession in central New York. The lithostratigraphic succession was determined through detailed mapping and logging of measured sections, provenance study of diamicton units, and the analysis of declination of detrital remanent magnetization (DRM) in laminated lacustrine silt and clay. The initial recession of southwest-flowing ice from the Adirondacks and at least two glacial re-advances of the Mohawk Lobe are recorded in these units. Glacial Lake Newport, impounded in the West Canada Valley during ice recession, attained levels that required ice-damming by coalescent Ontario and Mohawk Lobes and a subglacial outlet across the Mohawk Valley to the Susquehanna drainage basin. Initial ice recession in the Mohawk Valley was mostly by calving of active valley lobes in deep-water (up to 350 m) embayments and not by regional stagnation and simple downwasting. A shift of ice flow from the Adirondacks to ice flow from the Mohawk Lobe marks the development of low-land lobes during pre-Valley Heads time before the complete uncovering of the Adirondacks. The later dominance of the Ontario Lobe in Valley Heads time may have been the result of (1) a reduced supply of ice to the Mohawk Lobe from a diminished Adirondack ice dome, (2) changing ice-flow conditions in Quebec associated with an eastern St. Lawrence ice stream and calving bay, and (3) nonsynchronous surging of the Mohawk and Ontario Lobes as these lobes became fronted by deep lacustrine waters at different times.

LATE GLACIAL AND EARLY POSTGLACIAL ENVIRONMENTS IN WESTERN NEW YORK

For 1 0 millennia prior t o 7,500 B.P., progressively diminishing ice sheets dominated the environmental history of the northeastern states and eastern Canada. Glaciation effectively obliterates evidence of prior environments. Accordingly, upon deglaciation, landscapes of western and central New York presented a tabula rasa, a slate wiped clean of evidence of preglacial and early glacial environments. Shrinkage of ice sheets implies amelioration of climate and therefore conveys a n assumption of increased habitability of the newly exposed surfaces. In fact, deglaciation initiates a sequence of responses, each dependent upon achievement of prior threshold conditions. Accordingly, habitat evolution is progressive and protracted. The penetration of man into a continental interior locale is more strongly conditioned by the lagging pace of biotic recovery following deglaciation than by direct climatic controls. This study is a review of the chronology of physical and biotic changes that developed a habitat suitable for the entry of hunting and fishing populations into central and western New York. The only part of New York that escaped glaciation was t h e Salamanca Reentrant, the northernmost embayment in the glacier limit (FIGURE 1). Even this crudely triangular area, defined approximately by the course of the Allegheny River in New York, did not completely escape t h e effects of glaciation. Plateau summits in the Salamanca Reentrant were subjected t o accelerated mass wasting and frost action. The Allegheny valley floor experienced valley train aggradation as well as local glacial invasion and glaciomarginal impounding of the Allegheny and some of its tributaries.22 As shown by MacClintock and Apfel,’ the northeastern border of the Salamanca Reentrant marks the limit of an earlier glaciation than that on its northwestern border. Even from this latter area, marked by the Kent Moraine, recession was well underway prior t o 15,000 B.P., for peat from a shallow bog a t the edge of a later recessional moraine near South Dansville yielded a radiocarbon age of 15,300 k 190 B.P. (Clarence Gehris, S.U.N.Y. College at Brockport, informal communication). Recession of the ice margin exposed a deeply dissected plateau with local relief of 150 t o 300 m. In the counties of the “southern tier,” along the Pennsylvania border, broad upland remnants retain an accordance of summit elevation that indicates the ineffectiveness of glacial scour near its southern limit.’ Valley configuration and drainage pattern are little modified, bearing close resemblance t o those beyond the glacial limit t o the south. Northward, however, glaciation was progressively more effective in altering the preglacial l a n d ~ c a p e . ~ Summits are rounded and reduced in elevation. Glaciation transformed stream valleys into typical glacial troughs characterized by hanging tributaries, U-shaped transverse profiles, truncated spurs, and smoothly curving valley walls. Glacial flow across divides scoured deep cols, developing a “through-valley” pattern that bears limited relationship to the preglacial valley

New York glacial geology, U.S.A.

Publisher Summary This chapter reviews that the limit of glaciation in the north-eastern United States lies in Pennsylvania beyond New Yorks southern border except along the coast of Long Island, in eastern New York, and in the Salamanca Re-entrant, in western New York, where relief is high and the southern limit of glaciation has its most northerly position, east of the Mississippi River. New York may be presumed to have experienced several glaciations, deposits of pre-Wisconsinan glaciations are preserved only in unusual situations. Where glacial deposits are inset within protected gullies, transverse to the main flow of glacier ice, older drift may have escaped erosion during later glaciation. It also reviews that patterns of ice flow during a glacial advance are largely controlled by pre-existing bedrock topography and relief. As the Laurentide Ice Sheet moved into New York State, the Adirondack Highlands obstructed the main flow so that the ice spread around the Adirondacks with a lobate front southward into the Champlain Valley and south-westwards over the Tug Hill Plateau into the Ontario lowlands. Eventually, the thickening ice sheet topped the Adirondack Highlands and continued its advance into southern New York and Pennsylvania.

The use of paleomagnetic declination to test correlations of late Wisconsinan glaciolacustrine sediments in central New York

Detrital remanent magnetization (DRM), recorded in laminated glaciolacustrine silt and clay of the western Mohawk Valley of central New York, was used to construct a secular variation record of geomagnetic declination for parts of the late Wisconsinan in the interval 12.5-18 ka B.P. Remanent declination has been used for time-stratigraphic testing of correlations between laminated fine-grained glaciolacustrine sediments at different exposures in the western Mohawk Valley. A discontinuity in the remanent-declination record supports the interpretation that an observed lithostratigraphic discontinuity in the region represents an unconformity. Diamicton units, composed of till and proximal proglacial diamictons, are overlain by laminated clay and silt units that have time-transgressive lower contacts. The clay and silt units exhibit stratigraphic onlap in the direction of ice recession, a characteristic that is recognizable in their paleo-magnetic records. The use of a secular variation record of remanent declination has an advantage over lithostratigraphic and morphostratigraphic correlation techniques in that it is time dependent. For a part of the late Pleistocene, it provides a powerful tool for correlating physically disjunct stratigraphic assemblages in New York and New England, where no other dating technique has provided interregional correlations with comparable resolution. Sampling of different exposures of contemporaneous sediment was used to demonstrate the fidelity of remanent declination as a recorder of geomagnetic declination. The remanent inclination record was not useful for testing correlations because inclinations in strata known to be contemporaneous were different. As compared to remanent inclinations from other contemporaneous lacustrine sequences in the eastern United States, the average remanent inclination of the western Mohawk Valley sediments is at least 11 degrees too shallow. Shallow inclinations may be the result of errors acquired at the time of deposition or compaction resulting from drying of the sediment, consolidation by overriding ice, or lithostatic load of overlying sediment. Subglacial deformation, dewatering, and some outcrop conditions, especially ground-water seepage, were recognized as processes that may cause or facilitate postdepositional disturbance of remanent magnetization.