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Genesee Valley Glacial and Postglacial Geology from 50,000 Years Ago to the

Present: A Selective Annotated Review

Richard A. Young, Department of Geological Sciences, SUNY, Geneseo, NY 14454

Introduction

The global chronology for The Pleistocene Epoch, or “ice age,” has been significantly

revised during the last three decades (Alley and Clark, 1999) as a result of the extended and more

accurate data provided by deep sea drilling projects, ice core studies from Greenland and

Antarctica (Andersen et al. 2006; Svensson et al. 2008), oxygen isotope studies of marine

sediments, and climatic proxy data from lake cores, peat bogs, and cave stalactites. These new

data have improved our ability to match the Earth’s Milankovitch orbital cycles to the improved

ice core and radiometric chronologies (ages based on radiocarbon, U-Th, U-Pb). However, the

Milankovitch theory has recently been the subject of renewed controversy, and not all cyclical

climatic phenomena are directly reconcilable with Milankovitch’s original ideas (Ridgwell et al.,

1999; Ruddiman, 2006). Overall, it is evident that there must have been as many as 20 or more

glacial cycles in the last 2.5 million years, not all of which necessarily resulted in the expansion

of large ice sheets as far south as the United States-Canadian border.

The International Union of Geological Sciences recently adopted a change for the

Pliocene-Pleistocene boundary, extending the beginning of the Pleistocene Epoch back from 1.8

to 2.588 million years Before Present (BP). The average length of the most recent glacial-

interglacial cycles (also known as “Stages”) is on the order of 100,000 years, with 10,000 to

15,000 years being the approximate length of the interglacial warm episodes between the longer

cold cycles (also known as cold stadials and warm interstadials). Although a wide variety of

geologic studies has improved the chronology of major terrestrial and oceanic climatic

oscillations during the youngest “late” Wisconsin Stage of the Pleistocene Epoch, there are still

unanswered questions concerning the causes of shorter climatic cycles such as Heinrich Events

and Dansgaard-Oeschger Cycles, as well as unique climatic excursions, such as the Younger

Dryas cold period around 12,000 years ago (Voelker, 2002).

The middle and late Wisconsin glacial history of the Genesee Valley (Figure 1) has been

extended back approximately 50,000 years by the dating of glacial and interglacial sediments, as

well as wood samples recovered from water well drill holes, and by a complex stratigraphic

record preserved in two adjacent gravel excavations on the west bank of the Genesee River near

its confluence with Honeoye Creek (Young and Sirkin, 1994; Young and Burr, 2006). The

glacial history has been extended back through a middle Wisconsin ice advance that occurred

approximately 35,000 years ago to a warmer interstadial episode that peaked globally around

50,000 years ago. This middle Wisconsin ice advance buried interstadial spruce trees growing

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on the ancestral Genesee floodplain that have finite radiocarbon ages as old as 48,800 14

C years

BP and extending slightly beyond the range of conventional radiocarbon dating (Young and

Burr, 2006).

The postglacial Genesee River floodplain history also has been extended back nearly

15,000 calendar years from analyses of wood samples buried by a prehistoric landslide near

Avon, NY (Young, 2003). Historic human impacts on the Genesee River floodplain at the

surface and from underground salt mining are significant and ongoing (Young, 2003). This brief

review cites critical peer-reviewed references that contain the details of the glacial history of the

Genesee Valley and of the adjacent Great Lakes and eastern North America, as well as some of

the most relevant concepts, chronology and climatic data that are essential to understand the

local glacial history within a broader global perspective.

Wisconsin Glacial History

Although the most recent Wisconsin Stage of the Pleistocene Epoch lasted approximately

100,000 years, most of the depositional glacial features (drumlins, moraines, kames, kettle lakes,

eskers, outwash channels, proglacial lake shorelines) that modify the older bedrock landscape of

New York State were only formed during the late Wisconsin glacial advance and recession,

between approximately 21,000 and 12,500 14

C years BP in upstate New York (see endnote on

radiocarbon dating conventions and calendar year conversion). The late Wisconsin glacial

episode essentially ended in New York State when the Laurentide ice sheet melted back from the

latitude of the glacial Lake Iroquois shoreline (Ridge Road) into the Lake Ontario Basin. Lake

Iroquois drained rapidly when the ice sheet finally melted back from the northern Adirondack

Mountains (Covey Hill spillway) and opened the St Lawrence Valley for glacial meltwater to

escape directly to the Atlantic Ocean, rather than following its earlier eastward routes through

the Hudson and Mohawk Valleys or westward into the glacial predecessors of Lake Erie.

Reviews of the Wisconsin glacial history of the eastern Great Lakes and eastern North America

can be found in Karrow and Occhiette (1989), Karrow et al. (2000), Muller and Calkin (1993),

Muller and Prest (1985), and Morner (1971), as well as in the numerous references therein.

During the final retreat of the Late Wisconsin ice sheet a series of 14 progressively larger and

lower proglacial lakes formed in the Genesee Valley (Fairchild, 1904, 1928), a sequence of

events that is reviewed and updated in Muller et al. (1988).

The last major glacial lake stage that submerged part of the lower Genesee Valley was

Lake Iroquois (~435 ft. ASL near Rochester), after which Lake Ontario dropped significantly

below its current elevation (246 ft ASL) to a level currently referred to as “early lake Ontario,” a

lake stage whose exact age and lowest elevation have been somewhat uncertain (Muller and

Prest, 1985). The surface water level of this early Lake Ontario stage at Irondequoit Bay was at

least 140 ft below current lake level, as documented by borings for the Town of Webster well

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Figure 1. General location map with outline of Genesee River Basin and key locations discussed in text.

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field, the Irondequoit Bay bridge, and the new bridge foundation borings located on the

Irondequoit Bay outlet sandbar (Young, 1983, 1988; Kappel and Young, 1989; Erdman et al.,

1991). Anderson and Lewis (2012, in press) have revised the water-level history for this low

water stage in the Lake Ontario basin. Their comprehensive analysis indicates that Lake Ontario

occupied a closed basin, disconnected from an outlet to the Atlantic Ocean for approximately

4,000 years beginning 12,300 calendar years BP during a relatively dry climatic interval.

Immediately prior to this time the lower lake level was confluent with the Champlain Sea, but

the fresh water influx to the lake prevented it from becoming a marine environment. During the

low, closed-basin stage, Lake Ontario’s shoreline would have been as much as 50 to 60 meters

(164 to 197 feet) below modern-day sea level.

Ongoing glacial rebound of the rising Ontario outlet threshold in the St Lawrence Valley

has gradually caused the southwestern Lake Ontario lake level to rise, creating the drowned

shoreline features (embayments) that characterize the lakeshore from Sodus Bay westward to

Buffalo (Muller and Prest, 1985; Young, 1983, 1998). In contrast, the northeastern shoreline of

the lake has been elevated (emerged), thus shaping modern features such as the Thousand

Islands. Contrary to some of the ideas of Fairchild and other early workers, marine waters

(Champlain Sea) never dominated the St Lawrence Valley further southwestward than the

approximate longitude of Brockville, Ontario, immediately after the ice retreated, and before

postglacial rebound of the land became significant. Differential postglacial rebound continues to

tilt Lake Ontario toward the southwest at a rate that changes the relative shoreline elevations by

almost one foot per century between the eastern and western ends of the lake.

The existing landforms in the Genesee Valley downstream from the postglacial

Letchworth Park gorge were formed by a combination of ice advance (erosion), ice recession

(deposition, including a widespread mantle of varved lake sediments and shoreline deposits), and

subsequent modifications related to the effects of the modern Genesee River (fluvial erosion,

floodplain deposition, landslides, historic deforestation, agricultural impacts, mining) (Young,

2003). The glacial landforms and glacial geology of the region and across the entire state may be

examined on the 5 large sheets of the Surficial Geology Map of New York published in 1986 as

New York State Museum Map and Chart Series #40 (Scale, 1:250,000).

Correlation of Glacial Terrestrial and Glacial Marine Records

The incomplete terrestrial chronology of continental glaciations has been gradually

improved and reorganized using the more complete and combined climatic records from ice cap

studies and from hundreds of deep sea sediment cores. The ability to correlate the Pleistocene

stratigraphic record from the deep sea cores directly with poorly preserved terrestrial landforms

(such as glacial moraines) is a more difficult and challenging task, given the sparse occurrence of

organic remains that are preserved in the generally coarser and more oxidized continental

sediments. The Genesee Valley near the Monroe-Livingston County border contains a unique

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and diverse continental record of organic remains that is contemporaneous with one of the best

studied marine glacial events in the North Atlantic Ocean, Heinrich Event H4.

Seven thin sand layers, named Heinrich Events, and designated as H0 through H6, have

been defined in North Atlantic Ocean sediment cores and are presumed to record seven distinct

periods of extreme Laurentide glacier advances, or surges, accompanied by massive episodes of

iceberg calving (Bond and Lotti, 1995). These massive iceberg discharges are tentatively

attributed to an unstable thickening of the Laurentide ice sheet, which eventually results in basal-

ice shear failure accompanied by a sudden outward surge of the ice margin. The result is a

massive, but relatively short-lived, discharge of icebergs into the North Atlantic (Bond et al.,

1992, 1993; Cortijo et al., 1997; Elliot et al., 1998). The southward drift and slow melting of

these iceberg armadas disperses sandy glacial sediment that forms thin but pervasive sand layers

encountered in the otherwise fine-grained sediments that characterize most deep sea sediment

cores. These Heinrich iceberg discharge events have been dated as occurring at the following

approximate times (14

C years BP): H0 at 12,000; H1 at 17,000; H2 at 24,000; H3 at 31,000; H4

at 35,000; H5 at 45,000; and H6 at 60,000. A number of the younger Heinrich Events

correspond to the approximate ages of the major dated glacial moraine sequences in the

terrestrial glacial record of North America (Clark and Bartlein, 1995; Mooers and Lehr, 1997).

In other words, the unusually prolific marine iceberg calving events correspond with terrestrial

ice sheet advances that created some of the major glacial terminal and recessional end moraines.

However, accurate radiocarbon dating of organic material is not feasible for events much older

than 49,000 14

C years BP, and few glacial deposits older than 35,000 in eastern North America

contain well-preserved and datable organic remains. Wood found in glacial tills generally has an

uncertain age relationship to the encompassing sediments, due to the fact that glacial advances

and associated till deposition can override and incorporate organic remains that are significantly

older than the age of the till-forming ice advance (Young and Burr, 2006; Young, 2003).

Heinrich Event H4 (35,000 14

C years BP) is marked by a relatively thick detrital sand

layer (2 to 4 inch range) in deep sea cores. There is evidence in the ocean cores that it was a

dual-pulsed glacial advance (sand layer with 2 distinctively coarser intervals). It is estimated by

marine geologists to be as short as 250 years in duration, but may have impacted global climate

for as long as 1000 years (Kirby and Andrews, 1999; Cortijo et al., 1997). The calendar age for

circa 35,000 14

C years BP dates most likely falls somewhere between 37,000 and 39,000 actual

calendar years BP.

Middle Wisconsin Site, Northern Livingston County

A complexly stratified glacial deposit in shallow gravel pit excavations adjacent to the

west edge of the Genesee River floodplain in northern Livingston County, opposite the Genesee

confluence with Honeoye Creek (Figures 1-3), was studied for over 10 years by the author

(Young and Sirkin, 1994; Young and Burr, 2006). The site produced 68 radiocarbon dates with

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ages ranging between 30,000 and 49,000 14

C years BP on large wood fragments, mammoth

bones, and miscellaneous organic materials (spruce cones, twigs, tiny pelecypod shells, small

plant roots) all of which predate the most recent (late Wisconsin) ice advance in western New

York. The late Wisconsin ice advance that created the terminal moraines in northern

Pennsylvania and on Long Island occurred approximately 22,000 to 20,000 14

C years ago.

Middle Wisconsin time in NY State is poorly defined but probably corresponds roughly to events

spanning the time interval from ~30,000 to ~60,000 14

C years BP.

Figure 2. Location map for Elam-DeWitt middle Wisconsin glacial site (pink) in northern Livingston County. Arrows indicate flow of younger, late Wisconsin meltwater in well-preserved ice-marginal channels shown as light blue. Modern flood plains of Genesee River and Honeoye Creek colored yellow.

The unique middle Wisconsin glacial section in the lower Genesee Valley consists of an

interstadial ancestral Genesee River floodplain deposit dominated by spruce trees between

49,000 and 36,000 14

C years old and overlain by two separate gray glacial lake deposits, all of

which underlie the red-colored glacial till of the better-known late Wisconsin ice advance

(Figures 4-9). The two middle Wisconsin lake deposits are very close in age (circa 35,000 14

C

years BP), but are separated by a conspicuous gravel deposit that signifies a short interval of

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glacial retreat and coarse glacial outwash deposition (Figures 4,5). The glacial lake sediments

contain distorted annual varves that were subsequently converted to more homogeneous glacial

tills (known as “deformation till”) by the two associated short ice advances that had formed two

consecutive proglacial lakes in the Genesee Valley (Elson, 1961). The two varved lake clay and

silt deposits were subsequently overridden and deformed by two southward ice advances

(Figures 4, 10). The close agreement of the ages of these two closely spaced ice advances with

the established age of marine Heinrich Event H4 is convincing evidence that the dual glacial

surges suggested by the marine sediment cores (two closely-spaced, coarse sand layers) were

contemporaneous with terrestrial ice advances well inland from the North Atlantic margin. Prior

to the publication of the details of this well-dated middle Wisconsin site in the Genesee Valley

by Young and Burr (2006), Canadian geologists had long debated whether any ice sheet of

middle Wisconsin age had ever advanced south of the latitude of Toronto (Hicock and

Dreimanis, 1992a, 1992b; Eyles and Eyles, 1983).

Figure 3. South-looking aerial view of Elam and DeWitt gravel pit sites in northern Livingston County.

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Other proglacial deposits in the Cayuga Valley and from a site closer to Buffalo

previously provided a few isolated 14

C ages for events close to the same middle Wisconsin

interval, but the glacial events associated with those sites are not as clearly defined or as well

documented as in the Genesee Valley study (Muller and Calkin, 1993). In summary, well

documented evidence now exists for a middle Wisconsin ice advance that extended at least 20

miles (32 km) south of the Lake Ontario shoreline and that clearly preceded the classical late

Wisconsin event that created most of the modern surficial glacial landforms. Additional

evidence from other buried organic remains and dated wood from engineering and water well

borings (Young and Burr, 2006) suggests that the record of middle Wisconsin glaciation is

probably much more widespread, but is buried beneath glacial drift in the scoured bedrock

valleys of the northern Finger Lakes region.

Figure 4. Diagrammatic section of middle Wisconsin glacial sequence showing the record of two glacial advances (gray varve deformation tills) surrounded by yellow outwash gravels above green ancestral Genesee River floodplain. Ages are in

14C years BP.

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Figure 5. Author standing on stratified interstadial floodplain deposits (A) of ancestral Genesee River that are overlain by first gray deformation till (B) and capped by middle Wisconsin outwash gravel (C) at Elam site. Brown colors indicate normal oxidation that tends to destroy organic remains. The gray color of the deformation till (B) indicates limited oxidation due to the fine grain size of the original lake clay sediment, and also accounts for the unusually complete preservation of the varied organic remains collected on site.

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Figure 6. Close-up view of the contact between the interstadial floodplain sands (bottom) and overlying gray deformation till. The first gray clay layer at the base of the lens cap is the individual fine-grained sedimentary deposit from the first year of proglacial lake formation that killed (drowned) local vegetation, which in turn, formed the black organic-rich spots seen within this basal varve. Varve layers above the lens cap were remobilized into a more homogeneous deformation till by ice overriding the lake sediment at around 35,000

14C years BP. See Figures 4 and 10.

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Figure 7. The contact between the interstadial floodplain sandy river gravel (bottom) and the overlying varved lakebeds at the base of lower deformation till. Note how the varves conform to the irregularities on the original floodplain surface and are draped over individual pebbles (center). This indicates that the lake-forming event involved slow sedimentation in a low energy environment as the advancing glacier dammed the ancestral Genesee River. The pebble near the center of the image is approximately two inches in diameter.

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Figure 8. Contact between interstadial floodplain sands (bottom) and the basal varve of the initial proglacial lake sequence. This image clearly shows the nature of the organic-rich basal varve (gray and brown layer with black spots near top of view). Vertical black root (center) is from a small reed-like plant that grew in the floodplain sediment before the 35,000 year old ice advance, similar to annual plant growth seen on modern Genesee River point bars (Diagrammatic location of roots is shown on Figure 4). The organic remains of both the basal lake varve and the root were radiocarbon dated to obtain the relative age of the initial glacial lake (ice advance) compared to the age of the preexisting floodplain. The youngest organic remains found in the floodplain sediment were approximately 1000 years older than the 35,000-year-old basal lake sediment, which is in remarkable agreement, considering the random nature of the geologic preservation and the equally random collection and dating methods.

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Figure 9. Fresh-looking, well-preserved nature of 48,000- year-old spruce wood collected from clay-rich till at Elam site. Natural wood colors rapidly turned black after a few hours exposure to air (oxidization). These old woody remains must have been buried in the older floodplain sediments, scoured by the advancing ice, and then released into the proglacial lake during the first ice advance. Water logged wood in such an environment would sink to the lake floor and be buried by sediment. Such conditions can be seen in modern glaciers where they advance across and erode older surface deposits. Camera lens cap in upper right indicates scale.

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Figure 10. Diagrammatic view of how an hypothetical ice advance creates a deformation till from varved lake clays (rhythmites) and causes inversion of radiocarbon ages (older wood pushed onto top of younger lake material). Darker gray layer being dragged by advancing glacier represents woody peat of floodplain deposits later scattered throughout till layers. “Rhythmites” is a non-genetic term that is used when it is not clear whether the layers preserved are actually produced by annual sedimentation events (storm activity in lakes also can sometimes create multiple varve-like layers). Annual glacial varves usually contain finer-grained winter layers alternating with coarser summer layers.

Late Glacial and Postglacial History of the Genesee Valley

Floodplain sediments such as those of the Genesee River usually contain a detailed

sedimentary record of the erosional and depositional history averaged over a large region. In the

absence of bogs or lakes, such fluvial sediments often contain the best local records of climatic

variability, archaeology, and tectonic history. Such histories are admittedly incomplete and

require sophisticated sampling techniques and detailed analyses to accurately interpret the

geologic history. The postglacial history of the Genesee Valley is preserved within

approximately 50 feet of fluvial sediments that cover the glacial drift within the confines of the

modern Genesee River floodplain.

Detailed studies of floodplain erosion and deposition rates for Monroe County and for the

US Army Corps of Engineers include 55 new 14

C ages on organic materials collected from

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within the floodplain sediments (Young 1997, 2003). The oldest dates in these floodplain studies

come from a prehistoric landslide on the east bank of the Genesee River near Avon, NY, that

occurred nearly 15,000 calendar years before the present (Figure 1) (Young, 2003). This

landslide buried small trees that are preserved within 2 feet of the current channel bed elevation

of the Genesee River (Figure 11a, 11b). The adjacent river channel is eroded into glacial varves

at this site. This demonstrates that the Genesee River had carved its channel through glacial drift

essentially to its present depth at the time of the landslide and only a short time after the late

Wisconsin ice sheet is known to have receded from the northern border of Livingston County.

This landslide site is on the northern edge of the large Fowlerville moraine complex that fills

much of the valley between Geneseo and Fowlerville (Figure 1). The river floodplain is quite

narrow (1000 ft.) through this long morainal reach due to the greater erosional resistance of the

glacial till exposed by the 1973 Oxbow Lane landslide (Young, 1973; Muller et al., 1988). This

broad, nearly featureless moraine created glacial Lake Geneseo (Figure 1), the last glacial lake

stage in the central part of the Genesee Valley (Muller et al., 1988). Lake Geneseo extended

from the edge of the Fowlerville moraine southward nearly to Dansville, and was probably no

deeper than 120 feet (Muller et al. 1988).

The age and depth of postglacial erosion at the prehistoric Avon landslide indicates that

glacial Lake Geneseo could not have been a long-lived feature. Glacial Lake Geneseo would

have drained as soon as the Genesee channel had eroded deeply enough to cause the landslide.

Thus the river must have eroded entirely through the Fowlerville moraine to its present level by

15,000 years ago, within a few hundred years of the age of the estimated ice sheet recession from

this latitude. Furthermore, the modern elevation of the river channel at the foot of the prehistoric

landslide indicates that erosion (lowering) of the channel bed over the past 15,000 years since the

landslide occurred has been minimal (~2 feet). This suggests that lateral reworking (not

downcutting) of fluvial sediment within the upper 50 feet of the floodplain has been the

dominant activity of the river since that time, as argued by Young (2003). This is probably due

to two major factors: (1) the Genesee River channel crosses two bedrock thresholds at Geneseo

and near York Landing, which restrict vertical incision of the channel, and (2) the impact of

glacial rebound (southward tilting of the entire basin) has acted to reduce the channel gradient

between the York Landing bedrock threshold and the bedrock elevation of the resistant Lockport

Dolomite, which forms the crest of the Upper Falls in the City of Rochester.

Results of the 2002-2003 Floodplain Study

The floodplain study contracted by the US Army Corps of Engineers for the Genesee River

downstream from Letchworth Park gorge concluded the following (from Executive Summary;

Young, 2003):

Of the three major geologic reaches identified in this 57-mile study, the 15-mile channel

reach on the agricultural floodplain located on the former glacial lake bed immediately below

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the Mt Morris Dam has experienced the most rapid changes in recent time. Changes along

the remaining 42 miles of channel to the north are relatively insignificant by comparison.

Channel changes within the Mt Morris-Honeoye Creek reach (42 miles) since 1942 include

meander migration rates of up to 28 feet per year, with an average of 4.75 feet per year for 45

locations. The downstream banks of north-trending, non-vegetated meander loops show the

fastest erosion rates.

Estimated floodplain overbank sedimentation rates on the natural, forested (?) floodplain

were relatively constant prior to European settlement at rates between 4 and 6 inches per

century. Post-European settlement sedimentation rates appear to have more than doubled,

but radiocarbon data are too scarce to provide adequate measurements for the last 300 years.

Lateral migration of the river channel in areas where the Genesee flows through glacial till

and where the banks are naturally vegetated shows little change since 1942. However,

significant prehistoric channel changes near major tributary junctions north of Fowlerville

are obvious from abandoned channel patterns still visible on aerial photography. These

include the reaches immediately upstream and downstream from Dugan Creek (see Figure 2),

Oatka Creek and Black Creek (see Figure 1). Tributary junction areas on the floodplain are

likely to be areas of sudden or discrete channel changes (avulsions) triggered by exceptional

flood events combined with debris dams.

Both the measured rate of vertical channel sedimentation and the inferred rate of lateral

channel migration, implied by the ages of the majority of point bars sampled, suggest that

both the overbank floodplain sediments and point bar deposits to the depth of the modern

channel base have been extensively reworked and deposited by natural processes within the

past 2500 years with an estimated error of ±20%.

The channel shows an inverted, abnormal gradual widening upstream between Geneseo and

Mt Morris, below the Mt Morris Dam. This atypical upstream widening may be related to

flood-control discharge practices and/or land-use practices. Local channel widening also

occurs erratically near major and minor tributary junctions. This localized widening may be

a response to decreasing channel depth where tributary bedloads are added to the main stem

Genesee channel bedload.

The 15-mile Geneseo-Mt Morris reach of the river channel lengthened by 1.36 miles (9.8 %)

in the interval from 1942 to 2002, upstream from a bedrock threshold near Geneseo. This

length change resulted in an addition of 2.72 miles (both sides of channel) of erosion-prone

banks, which contribute to increased sedimentation along downstream reaches.

At least one conspicuous charcoal horizon (forest fire?) sampled between 5- and 6-foot

depths in the overbank sediments appears to correlate with a documented anomaly in global

climate that is preserved in drought records circa 536 AD on most continents. Location of

additional evidence of such potentially widespread events can provide a means of

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subdividing the floodplain stratigraphy into more geologically significant intervals over

larger areas, or to link similar chronologies among several basins.

Where the river banks are heavily vegetated, the dense root-growth exposed along the

channel sides attests to the erosion protection provided by natural tree growth along channel

banks.

The dating of annual leaf mats within point bar sediments provides a more reliable measure

of sediment age than ages derived from random wood fragments, which may have a complex

history of reworking from older buried logs. In contrast, charcoal residues from overbank

sediments do appear to provide reliable measures of sediment age (presumed floodplain fire

events).

Erosion by the Genesee River channel to near its present grade appears to have occurred in a

short time interval following the last glacial retreat. This assertion is based on the discovery

of a 14,000-to 15,000-year-old landslide exposed at the base of the modern channel near

Avon, NY. It is also assumed that the river had vertically incised its channel through the

Fowlerville moraine complex (immediately upstream) by this time. In any event, the

location of the landslide next to the modern channel indicates that the river must have been at

or very close to its present grade between Avon and Rochester by 14,000 years ago.

Mean annual discharge at the Mt Morris gage shows an increase of as much as 25% for the

period of record (1909 to present). If this is simply a normal fluctuation in the natural

system, it fortuitously coincides with the period of post-dam channel adjustment. Most of

the increased discharge postdates the regional drought of the mid 1960s.

Insufficient information exists at present to adequately address the potential contribution of

the 1994 Akzo-Nobel mine collapse and resulting floodplain subsidence to localized flooding

and recent channel adjustments between Mt Morris and Geneseo. Future studies should

address this issue.

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Figure 11a. Prehistoric landslide deposit on Genesee River bank near Avon, NY.

Figure 11b. Close-up view of trees buried by landslide in 11a. Red arrows are 6 inches apart.

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Summary and Conclusions

The geologic record of the glacial history of the Genesee Valley and west-central NY is

imperfectly understood and is more complex than indicated in much of the published geologic

literature. The known 14

C ages of major glacial features in central New York State is still very

limited in most areas, and the presumed ages of many events, such as ice advances marked by

dated moraine positions, have previously been extrapolated entirely across NY State from the

eastern Erie Basin near Buffalo all the way east to the Hudson Valley region (Calkin, 1970;

Cadwell et al. 1988; Muller and Cadwell, 1986). It is now clear there was a middle Wisconsin

ice advance into central and western NY circa 35,000 14

C years BP that extended well south of

the Lake Ontario shoreline, but the event has been poorly documented elsewhere. The presence

of these much older glacial lake deposits (varved clays) in the Genesee Valley also may have

complicated previous studies of the glacial history that were based on the paleomagnetism of the

varved glacial clays (Brennan, 1988). Discrimination between middle and late Wisconsin glacial

lake sediments within or near the floodplain would have been problematic prior to discovery and

dating of the Elam-DeWitt site. The middle Wisconsin proglacial lake sediments could have

extended well south of the middle Wisconsin ice margin, and exposures of varved sediments in

riverbanks or small tributaries might have widely differing ages (middle and late Wisconsin) due

to their random preservation and selective exposure by river and tributary erosion.

The preservation of the middle Wisconsin glacial sediments at depths no greater than 30

feet below the late Wisconsin surface in the Genesee Valley near Honeoye Creek also indicates

that the well known late Wisconsin ice advance southward into Pennsylvania, and as far as Long

Island, did not necessarily scour all the Finger Lakes valleys completely down to bedrock as

implied in Mullins and Eyles (1996). The shape and depth of the buried bedrock valley under

the Genesee River floodplain is similar to that of the largest Finger Lakes, and the longitudinal

valley profile is similarly influenced by the resistance of the southward-dipping resistant

Onondaga limestone. The buried valleys of the Finger Lakes region are likely to be hiding a rich

source of geologic information concerning middle Wisconsin, and possibly older glacial events.

The failure of the late Wisconsin ice sheet to significantly scour the middle Wisconsin glacial

drift from the buried Genesee Valley at the latitude of Honeoye Creek raises fundamental

uncertainties regarding our understanding (or assumptions) concerning the severity of glacial

erosion by individual ice advances, especially in the classic Finger Lakes region.

The postglacial history contained in the floodplain sediments of the Genesee Valley

provides a wealth of information concerning postglacial erosion and sedimentation rates, as well

as about the apparent historic impact of agriculture and dams on the behavior of large river

systems (US Army Corps of Engineers, 1969; Mansue et al., 1991; Mansue and Bauersfeld,

1991; Young, 2003). However, the impact of the Akzo-Nobel salt mine collapse on the Genesee

River’s hydraulic readjustment has yet to be quantitatively evaluated over the long term (Nieto

and Young, 1998). Much data concerning this avoidable mine disaster is contained in both

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published and unpublished reports, including the ongoing impact of upward vertical brine

migration into the basal floodplain aquifer near the collapse area east of Cuylerville, NY (Yager

et al. 2001, Nieto and Young, 1998). In addition, details of the subsurface geology of the valley

are as yet to be quantitatively evaluated from several detailed deep borings between Sonyea and

Fowlerville completed during the summer of 1999 (Research in progress, Young, 2011).

End Note: Radiocarbon Dating Issues

The problem of converting measured radiocarbon years to actual calendar years, using methods

such as tree ring calibration and continuous ice core records, is relatively accurate back to 30,000

radiocarbon years, but at that point calendar years may be as much as 5,000 years older than

measured 14

C ages. When reading the geologic literature it is important to distinguish between

radiocarbon years, commonly expressed as “14

C years BP,” (BP = Before Present) and “calendar

years” (actual years before the present, and usually expressed as “kyr BP”). The “present” is

always measured from 1950 AD, the common conventional reference point used for comparing

radiocarbon measurements (Joris and Weninger, 1998; Voelker et al., 1998). Calibration

programs to explain and readily convert radiocarbon ages to actual calendar years are maintained

online by radiocarbon research laboratories. One such program is CALIB, provided by the

Quaternary Isotope Lab, University of Washington, and currently maintained by Paula and Ron

Reimer. Maintenance of CALIB is supported by the University of Washington, Queens

University of Belfast, and the National Science Foundation. CALIB can be located by a simple

web search or accessed directly at: http://calib.qub.ac.uk/calib. The conversion of radiocarbon

years to calendar years is not a simple linear relationship, and individual radiocarbon ages

sometimes have two or more different calendar year conversions. Many interesting examples

and discussions of the relationships between calendar years and radiocarbon measurements for

different portions of the 50,000-year timescale can be found online. Regardless of these inherent

problems, 14

C dating is the best means of comparing the relative ages of glacial events that

happened during the past 50,000 years.

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