C10: SELECTED TILL AND STRATIFIED DRIFT DEPOSITS BETWEEN GLEN'S FALLS AND AMSTERDAM, NEW YORK
Donald T. Rodbell
Geology Department
Union College
Schenectady, NY 12308-2311
rodbelld@union.edu
INTRODUCTION
This trip is intended to provide participants with a review of the glacial stratigraphy of the portion of eastern New York State between Glenn's Falls on the northeast and Amsterdam on the southwest, and to correlate the regional stratigraphy with the better dated sequences of central New York. All of the data presented here were gathered by students in four different classes of Glacial and Quaternary Geology (Geo 52) at Union College between 1995 and 2002.
The region covered by this field trip contains a wide variety of glacial deposits-from subglacial till to subaqueous fan deposits to varves to eolian sands. Perhaps what is most interesting about this region is the evidence present for a dynamic history of ice marginal advances and retreats, and the numerous glacial lakes that occupied much of the region during the latter third of marine isotope stage 2 (Martinson et al., 1987). While there has been a plethora of prior work conducted in the area, a near absence of numerical age control coupled with stratigraphic uncertainties generated by a laterally discontinuous and variable lithostratigraphy provides us with considerable room for debate over the details of the glacial history of the region.

C10: SELECTED TILL AND STRATIFIED DRIFT DEPOSITS BETWEEN GLEN'S FALLS AND AMSTERDAM, NEW YORK

STOP 1. Basal Till of Luzerene Gorge and Multiple Diamicts
This stop is on the north side of the Luzerne Gorge, in sight of the Hudson River. Exposed along the north side of the Gorge are a thick sequence of diamicts, sand, gravel, and varves (Stop 2). At this stop we will examine the basal till of the Gorge. In addition to a strong NNE-SSW fabric (Fig. 2), this till yields composition indicators that reflect a provenance in the Hudson-Champlain lowlands and from regions underlain by crystalline rocks.
Magnetic susceptibility, carbonate content, texture, and percentage crystalline clasts provide a means to distinguish till deposited from Hudson-Mohawk ice from that deposited by Adirondack ice. Tills deposited by Hudson-Mohawk ice contain >3% carbonate, whereas Adirondack tills are devoid of carbonate (Fig. 1). Similarly, the matrix of tills deposited by Hudson-Mohawk ice contain >10% clay and yield relatively low magnetic susceptibility values (</200 x 10sup-3/sup SI units). These trends are most obvious when comparing tills in the eastern Mohawk Valley (e.g., Stops 1-1, 2- 1, 2-2 in Fig. l) with tills in the Adirondacks (e.g., Stops 5-1, 5-3 in Fig. l). The tills of the Luzerne Gorge have compositions that are intermediate between these two end members; nonetheless, these tills are clearly distinguished from Adirondack till.

STOP 2. Varves Between Upper and Lowere Tills of Luzerene Gorge
Here along this portion of the roadcut are exposed ∼ 10 meters of varves. These are stratigraphically above the basal till of Stop 1 and below the uppermost till, which is exposed along the roadcut several hundred meters up the hill to the southwest of this outcrop. The site has been slumped and colluviated for years, but we have excavated through the colluvium and found varves at nearly all levels of this outcrop. These lake sediments may have been deposited in the southern-most segment of Glacial Lake Warrensburg (Connally and Sirkin, 1971), or in a smaller lake restricted to within the Gorge.
The presence of lacustrine sediment between diamicts reflects an interstadial period. In order to estimate the duration of this interstadial in the absence of a clean and continuous outcrop of varves, we have over the years determined sedimentation rate (couplets/cm) for varves in several positions of the outcrop. Typical values are 3.5 couplets/cm. If this rate is representative of the average sedimentation rate for the entire outcrop, then the duration of this interstadial period was ∼3500 years. Owing to the presence of several thick sand layers within this varve sequence, the calculated duration is best considered to be a maximum limiting duration.
Correlation of this interstadial period with the regional record of glaciation is tenuous, but it seems likely that the interstadial recorded here correlates with the Shed Brook Discontinuity in the western Mohawk Valley (Ridge, 1997) and thus with the Erie Interstade (14-16 ka). As noted above, the entire sequence here must be older than 13,150 14C yr BP, and this is consistent with correlation to the Erie Interstade.
The uppermost till is not very accessible in the Gorge, but a colluviated section of it is exposed about 200 m to the southwest of Stop 2. The till there is compositionally very similar to the basal till, again implying an origin in the Hudson-Champlain lowlands.

STOP 3. Delta on North End of Sacandaga Basin
This site, which is located on the north side of the Great Sacandaga Lake, exposes more than 10 m of well-sorted sand and some gravel, in discrete foreset beds that dip southward. Superimposed on these beds are well-preserved Type A climbing-ripple cross-laminations (Harms et al., 1982) that are exposed in planes parallel to flow. Both the foreset beds and the climbing ripples indicate a strongly southward paleocurrent direction (Fig. 4); or into the Sacandaga Basin. This deposit represents a delta or kame delta sufficiently distant from the ice margin to exclude the coarse fraction of glacial outwash.
A delta with southward paleocurrent directions on the north side of the Sacandaga Basin requires a lake in the basin, much as exists today. The elevation of this lake surface at its highest was 300 masl (∼985') based on the elevation of the topset-foreset transition of a large kame delta at the mouth of the Sacandaga River exposed in a gravel pit ∼3 km northwest of Northville.
To generate a lake of this elevation in the Sacandaga Basin, the outlet of the Basin near Conklingville or Lake Luzerne must have been dammed by the Hudson-Mohawk lobe. In addition, to generate a lake level of ∼300 m, the southern margin of the Sacandaga Basin must also have been dammed by the Hudson-Mohawk lobe. The top of the Broadalbin-Interlobate moraine (Stops 4 and 5; Dineen and Hansen, 1995), which is ∼20 km south of the aforementioned highest foreset beds has an elevation of ∼260 m. Even a high gradient of isostatic tilting of ∼1 m/km is insufficient to make the BIM a viable dam for Glacial Lake Sacandaga; the surface of the Hudson-Mohawk lobe must have been at least 20 m higher than the surface of the BIM. The presence of Glacial Lake Sacandaga requires substantial retreat of the Adirondack lobe while the northern side of the Hudson-Mohawk lobe at the BIM remained relatively stationary.

STOP 4. Herba Sand and Gravel Pit
This site is located on the northern edge of the BIM, ∼5 km west of Broadalbin. It exposes more than 25 m of stratified drift, which ranges in grain size from sand-cobble. Dineen and Hansen (1995) reported the presence of lodgement and flow till at this site. The dominant lithology here is metamorphic-mostly granitic gneisses.
Much of the BIM is underlain by stratified drift, and it seems likely that the BIM was deposited as a kame into standing water between the Adirondack lobe to the north and the Hudson-Mohawk lobe to the south. The distance between these lobes must have increased as Adirondack ice retreated northward and the northern edge of the Hudson-Mohawk lobe remained more-or-less stationary to the south of the BIM. This scenario is supported by paleocurrents which vary widely but which are dominantly southwestward (Fig. 4), along the axis of the BIM - thus meltwater was apparently transporting sediment off the sides of both lobes, but these currents were eventually shunted southwestward toward what must have been an outlet to Glacial Lake Sacandaga.

STOP 5. Broadalbin Town Gravel Pit
Located near the center of the BIM, this site exposes a complex stratigraphy of outwash sands and gravel, horizontally laminated lacustrine sands, all of which is overlain by a diamict. This latter has many of the hallmarks of Adirondack till; namely, a high magnetic susceptibility, an absence of carbonate, low % clay, and high % crystalline rocks (Fig. 1). In addition, this till yields NNE-SSW till fabric (Fig. 5), consistent with Adirondack ice.
The stratigraphy appears to record an advance of the Adirondack lobe from N-S over the BIM. However, Dineen and Hansen (1995) have interpreted the stratigraphy here to reflect an advance of the Hudson-Mohawk lobe (Yosts Readvance). The presence of rotten shale clasts supports their interpretation, and it is possible that the apparent Adirondack composition reflects Hudson-Mohawk flow lines that overtopped the Kayaderorseras Range ENE of this site. In this case the, it is possible that this till is correlative with the upper till in the Luzerne Gorge (Stop 1), and reflects a Luzerne Readvance that was considerably larger than envisaged by Connally and Sirkin (1971). If so, then the Mt. McGregor moraine of Connally and Sirkin (1971) either represents a younger readvance or a recessional ice position during deglaciation from the Luzerne Readvance.

STOP 6. West Glenville Gravel Pit
This site exposes a kame delta on the northern edge of the Glenville Hills. The deposit is about 20 m thick and contains sand and gravel beds that are inclined at about 20° to the west. This west-southwest flow direction (Fig. 6) likely reflects glacial meltwater off the western side of the Hudson-Mohawk lobe after it had shrunk to open a basin to the west. The elevation of this lake was ∼215 m (700') based on the elevation of the top of this kame delta, and on the elevation of numerous sand and gravel pits that demarcate the southern extent of the lake along West Glenville Road to the west of this locality.
Although LaFleur (1965) maps this kame delta at the eastern edge of his "Early Lake Amsterdam", it seems more plausible that this kame was deposited into a much smaller, restricted lake. This smaller lake would have been dammed to the east by the Hudson Mohawk lobe, to the south by the Glenville Hills and to the west and north by bedrock hills within 2 km of this locality. The most likely outlet for this lake was southward down Wolf Creek Hollow and Hoffman's Delta (Stop 7) to the Mohawk River. This is consistent with the elevation of the outlet to Wolf Creek Hollow (∼215 m), and explains: 1- why Wolf Creek Hollow is devoid of till, and 2- why Hoffman's Delta is so much larger than other deltas in the eastern Mohawk Valley. Proglacial lake drainage down Wolf Creek Hollow would have stripped the valley of drift and induced considerable down cutting. Much of this drift would have been deposited in Hoffman's delta.

STOP 7. Hoffman's Delta
Hoffman's delta is the largest delta in the Mohawk Valley east of Amsterdam. It was deposited at a lake level of ∼130 m (∼420'), and according to LaFleur (1965) it was deposited into a late phase of Lake Amsterdam, when the Mohawk Valley was plugged to the east, at approximately Scotia, NY, by the eastward retreating front of the Mohawk lobe.

STOP 8. Auiresville Exposure
Here are exposed 40 m of diamicts, gravel and sand. The base of the exposure is marked by a typical lodgement till from the Mohawk lobe that is at least 12 m thick. It contains several lenses of gravel, which may reflect intervals of meltout till. This till is overlain by at least 12 m of sandy colluvium, which appears to be derived from a sand and gravel unit that is at least 4 meters thick. This gravel unit possesses strata that dip northward at ∼20°. The top of the exposure is marked by a second till, at least 6 meters thick, which is oxidized at its surface, and which is, in turn, overlain by well-sorted fine-medium sand. The sequence appears to reflect multiple ice advances over the site. The two tills are nearly identical in composition, and clearly reflect a Hudson-Mohawk source (Fig. 1). What few fabric data we have from the lower till indicate that it was deposited along an E-W (or W-E) flow path (Fig. 7). The sand and gravel unit separating the tills appears to be deltaic and indicates the presence of a glacial lake in the Mohawk Valley at an elevation of ∼85 m (280').