4: DEGLACIATION IN THE SOUTHEASTERN LAURENTIDE SECTOR AND THE HUDSON VALLEY – 15,000 YEARS OF VEGETATIONAL AND CLIMATE HISTORY
Dorothy Peteet
NASA/GISS and Lamont Doherty Earth Observatory
Palisades, NY 10964
John Rayburn
SUNY New Paltz
New Paltz, New York 12561
Kirsten Menking
Vassar College
Poughkeepsie, New York 12604
Guy Robinson
Fordham College
Fordham, New York 10458
Byron Stone
United States Geological Survey
Connecticut 06340
Introduction
In this field trip, we provide a review of the significant controversy concerning the timing of deglaciation in the Hudson and Wallkill Valleys. We outline the differences in methodology and chronology with a circular route throughout the Hudson and Wallkill valleys. We begin the trip at Lake Mohonk near New Paltz led by Kirsten Men-king and Dorothy Peteet, then continue to the "black dirt" region of the Wallkill Valley where John Rayburn has contributed a new GIS model of deglaciation in the Wallkill Valley and Guy Robinson will review the history of fossil mammals, including mammoths. From this point we travel southeast to a rare exposure of glaciolacustrine beds on the west side of the Hudson River, described by Byron Stone and John Rayburn, and on to Croton Marsh at Croton Point, New York where Dorothy Peteet will review the marsh histories of the region.
A recent review of literature relating to the last glacial recession in the Hudson Valley indicates that the timing of deglaciation is very controversial (Peteet et al., 2006; Peteet, in review; Balco et al., 2006; Balco et al., 2009; Schaefer, 2007). Some questions to consider:
1) How does timing of new lake basal dates at the margin of the ice (Staten Island) compare with sites to the north and inland (ie. Mohonk)?
2) What is the vegetational history of the region and how does it compare with Deevey's classical southern New England stratigraphy?
3) What is the latest model of the deglaciation of the Wallkill Valley?
4) What have the Hudson marshes added to our understanding of the vegetation and landscape history, particularly in the last few millennia?

4: DEGLACIATION IN THE SOUTHEASTERN LAURENTIDE SECTOR AND THE HUDSON VALLEY – 15,000 YEARS OF VEGETATIONAL AND CLIMATE HISTORY

STOP 1. Mohonk Lake
We begin our trip with a walk around the shores of Mohonk Lake, guided by Paul Huth, director of research for Mohonk Preserve. Starting in front of Mohonk Mountain House, a Victorian era hotel built in 1869 and operated continuously by several generations of the Smiley family, we will climb onto the quartz pebble conglomerate cliffs of the Silurian Shawangunk formation that surrounds the lake. Glacial striations and crescentic gouges are evident at several points along the trail, and Mohonk Lake itself is thought to have been formed by glacial plucking of tectonically fractured bedrock. The sediment core described earlier in this guide was taken at the northern end of the lake near the Mountain House and spans most of the Holocene (see Figures 1-3). The first appearance of organic matter in the core dates to approximately 11.6 14 C kyr b.p., in keeping with young AMS dates found at other sites in southern New York and New Jersey. In addition, the core appears to record a mid-Holocene drought episode that might have caused the decline of hemlocks previously attributed to a pathogen.
Continuing along the trail affords a view of Rhododendron Swamp from above. A paleo-indian rock shelter adjacent to the swamp contains evidence of human habitation as early as 10 kyrs b.p. and possibly as early as 11.5 kyrs b.p. (Eisenberg, 1991). Ongoing paleoecological stratigraphy attempts to link the Swamp record to the archeological record..

STOP 2. Dutchess Quarry Caves
Paleo-indian culture and Pleistocene Megafauna. Perhaps the most significant early human site in the northeast is at Dutchess Quarry, in a group of small caves formed in Paleozoic limestone on the northwest side of Mount Lookout in southern Orange County, NY. One complete and four partial Paleo-indian fluted projectile points have been recovered from cave numbers 1 and 8 (Funk et al., 1969; Funk et al., 1970; Kopper et al., 1980; Funk and Steadman 1994) Bones of caribou, (Rangifer tarandus), extinct flat-headed peccary (Platygonus compressus) and the extinct giant beaver (Castoriodes ohioensis) have been among the 71 species of vertebrates discovered, although there is no clear association between any of the Pleistocene fauna and the cultural material (Steadman, Stafford and Funk 1997).
At an elevation of 177m Mount Lookout offers a view from 80m above the Black Dirt agricultural region, itself lying over the largest accumulation of terrestrial peat in eastern United States after the Florida Everglades. A succession of proglacial lakes occupying the Wallkill River Valley left a large, poorly drained area that was to become an immense peat deposit continuing to build throughout the Holocene. By the early 20th Century, these mucklands were being artificially drained for agriculture. Occasionally, maintenance of drainage ditches exposed the remains of the extinct Pleistocene megafauna. In this way, numerous mastodons (Mammut americanum) and at least three stag moose skeletons (Cervalces scotti) have been discovered in and around this vast wetland. The two most recent of the stag moose have been AMS dated to 12,180+/-60 and 11,040+/- 110 the latest known occurrence of this species in North America.

STOP 3. Newburgh Terrace
The only available exposure of glaciolacustrine deposits on the west side of the Hudson River is in this pit at the mouth of Moodna Creek on the south side of Newburg. The pit is in the south side of the excavated terrace that has a surface altitude of over 49 m (160 ft). A small inset plain at the top of the exposure had a surface altitude of just above 33 m (100 ft). The pit exposes: 1) cobble gravel at the surface, 2) glaciolacustrine beds that extend to the bottom of the cut. The gravel, 1-2 m thick, underlies the inset terrace surface cut into the southern part of the original higher glacial terrace landform. The gravel rests on a sharp, horizontal disconformity with the underlying sand. The surface of the gravel plain is correlated with nonspecific levels of glacial Lake Albany, projected from deltas surfaces and topset-fore-set contacts from deposits east and north of this site. The gravel in this exposure is a thin, postglacial fluvial terrace deposit, probably graded to lowering lake levels in the valley.
The glaciolacustrine sand deposit consists of alternating, laterally continuous beds of fine to coarse sand, pebbly sand, and very fine sand and silt. Basal bedding contacts are sharp; bedding forms are chiefly flat thin beds and laminations, with few ripples and hummocky forms. Color of beds is related to their composition and, therefore, their particle size: coarser beds are lighter colored and contain quartz, some feldspar, minor carbonates, and scattered pebbles of carbonate and sandstone. Darker beds are finer grained and contain platy rock fragments of shale and some carbonate. The lake beds have a gentle southerly slope of <1o reflecting their subaqueous origin as bottomset beds of an ice-marginal delta deposited in glacial Lake Albany.
The excavation exposed on this trip affords us an opportunity to determine the local vertical successions of delta bottomset underflow deposits, their cross-cutting relationships, and paleocurrent flow directions.

STOP 4. Croton Marsh.
Archeological evidence indicates that the Croton region was populated by the Kitchawanc tribe as early as 5000 BC. The marsh was called Senasqua, Croton itself is believed to be named for the local Indian Chief, meaning "wild wind". A 1718 census of the area counted 91 inhabitants in the Manor estate of Cortland, including Dutch settlers and English Quakers (http://www.crotononhudson-ny.gov). An overview of the Croton Marsh remaining from the landfill road to the south of the entrance to Croton Park. Today, one can only see the invasive Phragmites communis covering the site, but prior to European impact it was comprised of a diverse sedge mixture. Peteet et al. extracted a 10 m marsh core and pollen/macrofossil work on the core is in progress.

STOP 5. Croton Moraine and Varves.
A summary of Croton Point Pleistocene glacial history is provided by Sanders and Mergurian (l994). It is as follows:
1) Several tills starting with gray-tan, then red-brown, then, after red outwash was deposited, another red-brown till, followed by the yellow-brown till. The red-brown tills contain erratics from the west side of the Hudson River whereas the youngest and oldest tills contain only rocks found on the east side of the Hudson River.
2) After the youngest of the tills had been deposited and the glacier responsible for it had melted away, the region was flooded. All the drainage from the Great Lakes flowed eastward through the Mohawk Valley and down the Hudson. Proglacial Lake Albany was backed up behind the natural dam of till at the Narrows. Deltaic sediments from the ancestral Croton River and possibly drainages to the north were deposited along the east shore of this lake. The water plane presumably stood at about elevation +70 feet (level of the flat terrace underlain by topset beds of the delta that coincide with the uppermost water level). To the west, the depth of water where the clay was deposited away from influence of the delta was 70 feet. The coarse browner clays probably represent the dark suspended load of the river(s). The light clays are winter deposits when river(s) experienced low- flow conditions and/or were shut down altogether because their waters froze solid.