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Floodplain Formation Processes and
Archaeological Implications at the Grand
Banks Site, Lower Grand River,
Southern, Ontario
Ian J. Walker
Department of Geography, University of Guelph, Guelph, Ontario, Canada N1G
2W1
Joseph R. Desloges
Department of Geography, University of Toronto, Toronto, Ontario, Canada
M5S 3G3
Gary W. Crawford and David G. Smith
Department of Anthropology, Erindale College, University of Toronto,
Mississauga, Ontario, Canada L5L 1C6
Processes of floodplain development and the record of Princess Point cultural occupation
(A.D. 500– 1000) were examined at the Grand Banks site in the lower Grand River of southern
Ontario. The Princess Point Complex of the early Late Woodland is significant because it
represents the first shift to horticulture in this region in which inhabitants made significant
use of floodplains. The floodplain of the lower Grand River has been constructed primarily
via vertical accretion of sediment in a low energy environment conducive to limited erosion
and slow burial of middle and late Holocene sediments. At this site, cultural materials are
preferentially preserved in two buried soils each corresponding to relatively stable periods
of valley infilling at or before 3200 B.P. and 1500 B.P. (14C years). Initial formation of the
floodplain and subsequent stability of the floodplain surface can be tied to middle Holocene,
and later, base-level fluctuations in Lake Erie. Understanding floodplain development is crucial in determining the linkages between settlement pattern and chronology, and, conversely,
the archaeological record in floodplain settings provides important contemporary data for
modeling floodplain geomorphological processes. q 1997 John Wiley & Sons, Inc.
INTRODUCTION
The Princess Point Complex of southwestern and south-central Ontario is identified by many researchers as the ancestor of later Iroquoian societies in this region
(Smith and Crawford, 1995). The Princess Point Complex is significant because it
represents the first shift to the northern Iroquoian subsistence pattern that differed
from its predecessors in having horticulture in addition to hunting, fishing, and
Geoarchaeology: An International Journal, Vol. 12, No. 8, 865– 887 (1997)
q 1997 John Wiley & Sons, Inc.
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collecting (Smith and Crawford, 1995, 1997; Crawford and Smith, 1996). The earliest
evidence for crops (corn: Zea mays) in Ontario is derived from sites occupied by
Princess Point peoples between approximately A.D. 500 and 1000 (Crawford et al.,
1997; Smith and Crawford, 1997). These early horticulturalists made significant use
of floodplains, particularly along the Grand River (Crawford et al., in press). The
functions of these lowland sites as well as site formation processes on the floodplain have been poorly understood. Elsewhere we have outlined the complexity of
issues surrounding Princess Point seasonality and scheduling in the lower Grand
River Valley (Crawford et al., in press). In this paper we focus on site formation
processes and our understanding to date of the dynamic fluvial geomorphic regime
that has resulted in the geoarchaeolgical record in a large floodplain site, Grand
Banks (AfGx-3).
Floodplains provided attractive settlement locales for prehistoric populations in
northeastern North America, particularly during the phase of initial corn horticulture between A.D. 500 and 1000. The relatively rich alluvial soils provided high
quality land for cultivation, while at the same time these locales allowed unimpeded
access to riverine resources such as water, fish, and shellfish. Some have argued
that floodplains, by their nature disrupted habitats, provided highly productive disclimax vegetation rich in diverse annual weedy plants (Smith, 1992). These characteristics would be ideally suited to human habitation. Indeed, floodplain occupations are common in the latter half of the 1st millennium A.D. in Ontario as well
as in neighboring Pennsylvania and New York (Stewart, 1994; Prezzano and Stepponaitis, 1992; Ritchie and Funk, 1973; Stothers, 1977; Stothers and Yarnell, 1977).
In Ontario, this pattern contrasts with that of the later Iroquoians, who more frequently settled in upland locations well away from major rivers. Sometimes the
only nearby water source was a spring or small creek.
This article evaluates the riverine context in which Princess Point cultural materials were deposited and the specific nature of preservation, disturbance, and
burial as it relates to the tempo and character of floodplain formation affecting the
Grand Banks site throughout the late Holocene. Two fundamental objectives of the
research can be defined. First, the relative role of vertical and lateral accretion
processes affecting the alluvial valley-fill is characterized in order to supplement
archaeological evidence for the likelihood of long-term and/or multiseasonal occupation. Within the formerly glaciated regions of central and eastern Canada,
floodplains are generally low energy systems, subject to slow lateral migration and
receiving small quantities of overbank sediment during the last 8000 – 10,000 years
(Nielsen et al., 1993). However, conditions vary locally, particularly in terms of
erosion potential, the rate of surface burial, and spatial variations in the degree of
disturbance of cultural materials. Thus, identifying potential stable occupation surfaces requires an understanding of floodplain development at the scale of both the
occupation site and the surrounding river valley (Blum and Valastro, 1992). The
second objective is to draw a connection between the physical and cultural changes
reconstructed from the site evidence that may be linked to a set of common environmental variables that contribute to an explanation of floodplain development
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processes and influence settlement patterns. Selected environmental variables include base level changes (Schumm, 1993), climate variability and flood history
(Knox, 1993), channel and valley fill materials (Lewin, 1992), variations in sediment
yield (Meade et al., 1990), and changes in vegetative cover of the contributing
watershed (Campo and Desloges, 1994).
GEOMORPHIC SETTING
Peninsular southern Ontario is dominated by small drainage basins that are characterized by short rivers (, 200 km long) that drain into either Lake Ontario, Lake
Erie, or Lake Huron. The largest basin in the region is the Grand River watershed
covering approximately 6700 km2. The river extends from its source area northwest
of Hamilton for approximately 150 km to its terminus at Lake Erie near Dunnville,
Ontario (Figure 1). Investigations over the last 20 years have identified more than
40 sites in the lower parts of the Grand River, where Princess Point artifacts have
been preserved in predominantly alluvial and shoreline settings (Crawford and
Smith, 1996). The majority of these are found downstream of the city of Brantford,
where the Grand River becomes less steep and wider.
In the lowest 50 km, the Grand River is primarily a single thread channel that is
sinuous to straight and frequently is confined by valley walls made of resistant
glacial sediments. Downstream of Brantford, river width varies from 75 to 200 m
and the channel gradient averages 0.0007 m m21, which is approximately half the
gradient above Brantford. Confinement of the river channel and the overall character of the floodplain are related to the Quaternary geology of the region. Repeated
glaciations have eroded and reworked sediments derived from the underlying Paleozoic carbonate, sandstone, and shale bedrock. Subglacial deposition at the end
of the last glaciation (ca. 12,000 B.P.) produced a 0 – 20 m veneer of silty lodgment
till. Several proglacial lakes occupied the area during final retreat of the latest
Wisconsinan ice (between 11,500 and 10,500 B.P.), resulting in an extensive cover
of glaciolacustrine clays over the till and bedrock (Ontario Geological Survey, 1985;
Chapman and Putnam, 1984). Water levels in Lake Erie were several meters below
current datum for much of the Holocene, thereby facilitating entrenchment of the
river valley as isostatic recovery progressed. This was followed by gradually increasing lake levels, which, at least in Lake Ontario, is known to have influenced
floodplain sediment accumulation rates and hence potential for occupation of
shorelines and river estuaries (Weninger and McAndrews, 1989).
Grand Banks Site (AfGx-3)
The Grand Banks site is located on the southwest bank of the Grand River near
Cayuga, Ontario approximately 35 km upstream from Lake Erie (Figures 1 and 2).
In this reach the river is confined vertically by limestone bedrock and laterally by
glaciolacustrine clay uplands. Floodplain development is restricted to wider sections of the valley, particularly near river bends where point bars have formed.
About 50% of the valley bottom is classified as alluvial floodplain with the remaining
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Figure 1. Location of study site and known Princess Points sites in the lower Grand River and Niagara Peninsula region of southern Ontario.
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Figure 2. Contour map of the Grand Banks floodplain area showing boundaries of the lateral bar,
sampling transects for topographic and sedimentological survey (straight lines), and area of the detailed
excavation. Contour lines are m a.s.l.
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Figure 3. Oblique aerial photograph of the Grand River looking upstream (northwest) towards Grand
Banks. Dashed line is approximate location of 20th century maximum flood limit.
area comprised of active river channel, glacial deposits, and bedrock. Grand Banks
is situated on a lateral bar that is approximately 750 m in length and averages 175
m in width (Figure 3). Site morphology consists of an even to slightly hummocky
surface that slopes gradually upwards towards the valley wall (Figure 2).
Prior to our research, a test excavation by David Stothers provided preliminary
background to the site. Stothers, during small-scale excavations in the river bank,
discovered the presence of three separate cultural horizons separated vertically by
layers of silt (Stothers, 1977:107). These horizons represented, in his view, Early,
Middle, and Late stages of Princess Point development. Stothers (1977) suggested
that the middle level at Grand Banks was older than the layer from which he retrieved substantial comparative data at the Cayuga Bridge site (AfGx-1), a few km
downstream. The artifact assemblage consisted of chert tools and flakes and cordimpressed pottery typical of the period. The only plant remain noted was a carbonized corn kernel. No animal remains were recovered from Grand Banks, but
grey squirrel, white-tail deer, black bear, woodchuck, beaver, possibly elk, unidentifiable bird, sturgeon, freshwater drum, a sucker, clam shell, and snapping turtle
remains were identified from Cayuga Bridge (Burns, 1977:301 – 306). This assemblage indicates a rather eclectic animal procurement pattern that included riverine
resources but that was not primarily riverine in focus. Although Stothers did not
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date any Princess Point material from Grand Banks, he obtained one radiocarbon
date from Cayuga Bridge: 1200 6 132 B.P. (cal. A.D. 610 [870] 990; Lab. # S-714).
METHODS
The interpretation by Stothers had not been assessed until detailed archaeological investigations began in 1993 near the present river margin of the Grand Banks
bar (Crawford et al., in press). Recent excavations were confined to a 38-m-long
area at most 16 m from the river bank, and in the general area of the excavations
conducted by Stothers in 1972 (Figure 4). Excavations proceeded by maintaining
spatial and vertical control through removal of sediment and artifacts from their
stratigraphic context in 1-m square units by trowel and shovel. Relatively thick,
undifferentiated strata were excavated in 10 cm layers. Cultural features such as
pits were treated as single units and all sediment and artifacts were collected from
them without further contextual subdivision. Approximately 25% of the excavation
fill has been processed by flotation for the recovery of plant and animal remains
as well as other artifacts. To date, about 65,000 artifacts, mostly pottery fragments,
stone artifacts and flakes, have been recovered from Areas A, B, and C (Figure 4).
In order to characterize properly the mode and timing of site formation, a mapping and sampling design was implemented that encompassed the entire bar. A
topographic map of the floodplain surface was constructed with a vertical resolution of 2 cm between sampling points. The mapping included a cross-sectional
survey of the river channel. Sampling points on the floodplain were established
along systematic transects perpendicular to the river thalweg. Transects were
spaced at approximately 50 – 80 m intervals each extending from river edge to valley
wall (Figure 2). Approximately 10 – 15 boreholes were established on each transect,
yielding a total of 150 sampling locations. The systematic sampling design captured
most of the topographic and sedimentological variability of the bar. At each sampling point an Oakfield auger was used to establish: (1) thickness of the alluvial fill
above refusal depth (mainly clay or gravel); (2) changes in texture, organic matter,
and carbonate content with depth; and (3) sedimentological properties of each unit
including complete grain size distributions and sedimentary structures (where possible). The auger data were supplemented with descriptions of six natural cut-bank
exposures.
RESULTS AND INTERPRETATION
Fluvial Hydrology
River channel morphology, floodplain development, and valley shape are influenced by the flow regime of the river (Knighton, 1984:87). Modern hydrometric and
hydraulic characteristics of the Grand Banks reach provide evidence for at least
contemporary associations of overbank flood frequency, sediment transport, and
surface erosion. In turn, this provides a context in which past flood events and
possible site disturbances might be analyzed. Hydrometric data are available for
two upstream gauges at Brantford and Cambridge (see Figure 1 for locations),
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Figure 4. Profiles of sediment properties taken from Area B including: (a) grain size; (b) % calcium carbonate; and (c) % organic matter. (d) Concentration
of artifacts versus depth (a totaled for all three areas). Shaded boxes show average thickness of two buried soils (PI and PII).
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which can be used in conjunction with cross-sectional survey data from the site.
Mean annual flow for the period of record (A.D. 1916 – 1995) is 57 6 14 m3 s21. The
largest flood on record is 1420 m3 s21 (mean daily basis) and was generated by
snowmelt runoff in March 1948. Since then, there have been generally lower peak
flows and reduced flow variability, which can be related, in part, to water management strategies of the Grand River Conservation Authority.
Floods with a recurrence interval of about 1.8 years produce bankfull conditions
at the two gauging sites (Walker, 1995). At the Grand Banks site this would be
equivalent to a flood magnitude of approximately 700 m3 s21. The combined gauge
data reveal floods of this magnitude or larger (mean daily basis) occurred at least
32 times between 1914 and 1990. So under recent hydrologic conditions, prior to
flow regulation in the 1950s, the annual probability of flooding that begins to spill
water over parts of this floodplain was 0.50. This decreased to 0.33 following progressive development of reservoirs in the upper basin. Snowmelt- or rainstormgenerated floods, large enough to submerge the bar to an average water depth of
1 m and initiate sediment transport near the site, have a probability of occurrence
of approximately 0.05. Floods of this magnitude last from 1 to 3 days. The duration
and magnitude of localized flooding caused by ice jams is difficult to predict.
Few direct studies of flood effects on the Grand River floodplain have been
conducted. Gardner (1977) examined erosion and deposition following a May 1974
rainstorm flood that was high-magnitude but short in duration. Cobble-gravel splay
deposits of up to 20 cm thickness were noted on several floodplain surfaces proximal to the main channel. Vertical accretion of fine silts and clays was notably
absent. A contributing factor to the reduced sediment carrying capacity of rainstorm-generated floods is the higher surface roughness present on the floodplain
when the summer and fall herbaceous plant cover is fully developed. For example,
water depths on the Grand Banks bar were in excess of 1 m during the 1974 flood
(Riley, personal communication, 1994), but deposition of sand and silt was sporadic.
A useful measure of sediment transport capacity, and thus surface erosion potential, within a river-floodplain setting is unit stream power (v) defined as
v 5 g QS/w
in which g is the weight density of water (9800 N m23), Q is discharge (m3 s21), S
is river energy slope (m m21) and w is surface water width (m). For a range of
discharges between bankfull and period-of-record maximum flow at Grand Banks
(bankfull; Q 5 700 m3 s21, S 5 0.0005, w 5 155 m: maximum; Q 5 1420 m3 s21, S
5 0.0007, w 5 310 m), unit stream power varies between 22 and 31 W m22. Nanson
and Croke (1992) have classified floodplain environments according to a general
range of energy types. In their classification, floodplains where v is between 10 and
60 W m22 are at the lowest end of medium energy environments (their type B3).
Floodplains in this group develop, following slow but progressive meandering of
the channel, leading to flat or gently undulating floodplains of vertically and later-
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Figure 5. Digital elevation model of the Grand Banks floodplain area showing the four distinct depositional zones. Dashed lines show probable pathways of overbank flows where water velocities would
be highest.
ally accreted sands and silts. Burial and limited surface erosion of the floodplain
surface is expected. A test of this classification for the lower Grand River is undertaken here by examining detailed stratigraphy of the floodplain within the river
reach, a scale commensurate with geomorphic processes that are likely to affect
habitability of the site (Stein, 1993).
Bar Stratigraphy and an Alluvial Facies Model
Stratigraphic data from the 150 auger holes at Grand Banks can be grouped into
four facies sequences representing distinct depositional environments. Figure 5
illustrates the Grand Banks surface topography and four generalized deposition
zones: the bar head, outer-middle bar, back chute or channelized inner bar, and
the downstream bar tail. Stratigraphic features common to each of the four zones
are summarized in generalized facies sequences of Figure 6. In each zone the basal
unit is a compact clay, varying between 10 and 20 cm thickness, found conformably
on either a gravel/cobble diamict or limestone bedrock (Figure 6-facies C). The
clay facies is very fine-grained (median particle size of 8 f) and well-sorted near
the upper contact. At the base it is much coarser with carbonate clasts up to 20 mm
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Figure 6. Facies sequences representing generalized stratigraphy in the four depositional zones marked in Figure 5. Phi scale is a logarithmic transformation of particle size d (mm) where f 5 -log d/log 2; sand-silt boundary 5 4f; silt-clay boundary 5 9f.
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Figure 7. Generalized representation of the thickness of the upper buried soil unit (PII).
in size (b-axis) and a high (ca. 15%) CaCO3 content. Organic matter content averages about 7%. The basal facies is consistently below water table forming a gleyed
(Munsell color of 2.5Y 6/2) calcareous mud.
Above the basal clay is a massive unit (Fm) comprised of greater than 90% silt
and clay. The median grain size varies between 5f and 7f (Figure 6). In the channelized inner bar a variation of this facies (Fl) contains more clay, is occasionally
laminated, and is probably influenced by both slope-wash from the clay-capped
valley walls and ponding of water following major floods. In the tail and outermiddle bar areas this unit grades into a distinct dark (10YR2/1), organically enriched, horizon of about 15 cm thickness (PI on Figure 6). P designators referring
to buried soils are applied following common practice in facies sequence nomenclature (Miall, 1992) with the first-formed lower unit designated PI followed by PII.
Organic matter content varies between 8 and 12%. This unit is frequently absent
from the bar-head and back-chute areas. Overlying this there is a lighter silty unit
with massive to very weak horizontal bedding (Fm). The sediment is slightly
coarser than the underlying units (median grain size 5 3 – 7f), sand content is
between 1 and 30%, and organic matter is everywhere less than 6%.
A second organic-rich and dark (10YR2/1 to 10YR3/6) unit (PII on Figure 6) is
found in all four facies sequences and averages 22 cm in thickness. The PII layer
is thickest in the outer-middle and downstream tail regions of the bar (Figure 7)
and is comprised of mainly silt (median size 5 6f) with less than 5% sand. Texture
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Figure 8. Distribution and thickness of the postsettlement alluvium (PSA) at Grand Banks.
was finest in the bar-tail region. Organic matter concentrations are relatively high
and range between 11 and 15%, while CaCO3 concentrations vary between 2 and
10%. The lower boundary of PII is less diffuse than the PI transition and often
exhibits reddish-grey mottling. This mottling indicates ferric accumulation, possibly from leaching, and alternating oxidizing and reducing environments associated
with a fluctuating water table (Courty et al., 1989). Where exposed in trenches and
cut banks, the upper boundary of PII is abrupt. Color, OM content, and the small
blocky structure common to both P units suggest that these are buried A horizons
developed on alluvial initial material.
The uppermost facies (Fm), found in all four regions of the bar, is lighter than
the underlying units (10YR3/3) and is comprised of weakly bedded to massive silt.
This facies averages 68 cm thick but thins considerably in the back-chute and
upstream bar-head areas (Figure 8). The median particle size of 3 – 4f is coarser
than all other facies observed. CaCO3 and OM concentrations averaged 8% and 9%,
respectively. Thin, fine sand partings were noted in cut-bank exposures, but the
sediment appears to exhibit few structural properties throughout the bar. The upper 20 – 30 cm of this unit over most of the bar surface was affected by plowing
prior to 1954 (Riley, personal communication, 1994), but disturbance since then
has been associated with livestock grazing.
In summary, the Grand Banks lateral bar is comprised mostly of massive to
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weakly structured silts of alluvial origin. Several facies unit boundaries are diffuse
in terms of color and texture, a feature common to low-energy floodplain settings
(Hayward and Fenwick, 1983; Ferring, 1992). The basal clays are gleyed due to
nearly continuous saturation, whereas the overlying facies are brown calcareous
alluvial sediments that incorporate two distinct buried A horizons. The upper
boundaries of the buried soil (P) units with the overlying sediments are abrupt and
uneven over the bar surface, indicating that stratigraphic layering, and not pedogenic horizonation, dominates the bar sequence. There is no B horizon below the
modern plow zone (Ap); thus the upper profile can be classified as a Cumulic
Regosol soil (Fluvent). A similar classification can be applied to the two buried soil
sequences. The lack of more advanced soil development represents a combination
of limited surface exposure before burial and possible chemical stability in the
carbonate rich materials. Chemical stability can occur in calcium-saturated sediments (clays in particular) with abundant free CaCO3. Under these conditions,
translocations in the sediment profile are inhibited by the formation of stable calcium compounds that flocculate or bond to mineral grains (Bridges, 1978). The
result is a relatively cohesive matrix whose chemical properties reduce leaching
and horizonation.
The thinner and faintly laminated sediments in the inner bar areas, the dominance of silt sized sediment, and the absence of well-developed sedimentary flow
structures elsewhere suggest that sediments are contributed via overbank flooding
and settling from suspension. Sediments are aggraded vertically instead of being
deposited in bed forms of a laterally migrating channel. Silty sediment may be
deposited across the bar surface, particularly at the higher Grand Banks sites, via
a diffusion processes which leads to vertical aggradation over time (Pizzuto,
1987).
Artifacts and Depositional Chronology
Archaeological excavations provide a localized, but much more detailed view of
the floodplain structure to complement the sediment core data. Three subareas,
Areas A, B, and C, were examined to provide detailed cultural, chronological, stratigraphic, and settlement information (Figure 4). Area A produced the deepest and
least disturbed occurrence of the upper buried soil (PII; 60 – 80 cm). Area C has no
obvious occurrence of either the lower buried soil (PI) or PII, although artifact
density is high between 25 and 55 cm below the surface. A complex series of post
moulds and shallow pits indicate occupation. Area B is a trench varying between
1 and 2 m wide and was excavated to clarify the relationship between the stratigraphy noted in Areas A and C. The details of this relationship are discussed elsewhere (Crawford et al., in press). In summary, Area A exhibits very little vertical
separation between the 3200 B.P. (14C years) and A.D. 500 – 1000 Princess Point
occupations, whereas these occupations are separated by a 100 cm layer of mostly
massive silt and fine sand in Area B. The shallower Area C occupations have been
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moderately disturbed by plowing while the upper buried soil (PII) in Area A does
not appear to have been plow-disturbed to any great extent. In Area B, the stratigraphy clarifies a local topographic anomaly that causes PI to rise to meet PII in the
northern 4 m of the trench. Plowing appears to have greatly disturbed the occupational levels in that portion of the trench as well. Some plowing has affected
parts of PII in the southern portion of the Area B trench.
Artifacts are concentrated in the upper and lower buried soils (PII and PI) (Figure
4). Extremely low densities of artifacts, mainly chert flakes and charcoal fragments,
occur in the alluvium between PI and PII and immediately below the lowest soil
(PI). In these nonsoil strata, the low density of artifacts and complete absence of
cultural features such as posts and pits indicate that the prehistoric occupations
are confined to PI and PII. A combination of localized scour and fill as suggested
by the uneven thickness of PII (Figure 7), and possibly limited bioturbation (1-cmlong burrows were noted in a few instances), is likely responsible for artifacts
occurring between PI and PII and just beneath PI. A mixture of Princess Point and
historic artifacts between the modern surface and PII is probably a result of recent
plough disturbance (Crawford et al., in press).
Samples analyzed for radiocarbon age are from Areas A, B, and C and are discussed in detail in Crawford et al. (1997). The ages for samples from Area A are all
from carbonized corn fragments recovered by flotation. These ages are for the
potentially earliest corn at the site. The age for a sample from Area B date is on
corn from a pit (Feature 210) with a large sample of corn and late Princess Point
pottery. The ages for samples from Area C were obtained to ascertain the age of
cultigen remains discovered there and to help sort out the ages of the complex of
posts and pits. Area C appears to contain the most recent archaeological materials.
They date to the late historic period when a Cayuga Iroquois settlement is known
to have existed on the Grand Banks floodplain (Faux, 1985). Two dates on corn
from Areas B and C are consistent with a terminal Princess Point age (cal. A.D.
1000 – 1030). The three ages from Area A of cal. A.D. 530, 570, and 780 are evidence
of earlier occupations, as well as the earliest directly dated maize in the northeast
(Crawford et al., 1997).
Radiocarbon dates suggest that bar development began prior to approximately
3200 B.P. (14C years) when aggradational conditions were favored. Floodplain sediments accumulate to a height above which the probability of additional sediment
accumulation is controlled by the frequency and geomorphic effectiveness of catastrophic, high magnitude flood events (Nanson, 1986; Ferring, 1992). The maximum bar height (thickness) will vary in response to major changes in flow regime,
which, in turn, depends on climatic, tectonic, and hydrologic factors over long time
intervals (Schumm, 1993). In the case of the Grand Banks site, it is suspected that
initial bar formation, thickening, and stability are closely tied to late Holocene
changes in Lake Erie water levels. These, in turn, occurred in response to differential isostatic adjustments of the land surface relative to lake levels particularly
near the outlet at Niagara Falls.
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Figure 9. Solid curve shows changing Holocene water levels for Lake Erie relative to the International
Great Lakes Datum (IGLD). Elevations of the modern, PI, and PII surfaces at Grand Banks are also
shown. Note that the development of PI begins after the probable Nipissing II high water event.
Base Level Changes in Lake Erie
The accepted modern datum for Lake Erie is 173.3 m above sea level (Bishop,
1987). Mean summer water elevation at the Grand Banks site is about 179 m a.s.l.,
or about 6 m above lake datum. Coakley and Lewis (1985) used studies of geomorphic and paleoecologic indicators to reconstruct the Holocene water level history of Lake Erie. Figure 9 shows the chronology of lake level changes relative to
modern lake datum. Between 13,000 and 12,000 B.P. the southern margin of the
Laurentide Ice Sheet was withdrawing from the lower Great Lakes. A series of
small subbasins in proto Lake Erie were partly filled to levels of around 30 – 36 m
below datum (b.d.). By 10,000 B.P. lake level rose rapidly to 15 m b.d., after which
the rate of level rise decreased dramatically. The rise in water level reflected two
factors: (1) isostatic uplift of the bedrock sill beneath the Niagara River outlet
relative to the west end of the lake and (2) increased water inputs due to changes
in climate, ice sheet melting, and drainage realignment in the upper Great Lakes.
By 7000 B.P. water levels were 5 m b.d. and remained nearly constant until 5000
B.P.
Coakley and Lewis (1985) argue that between 5000 and 3900 B.P. Lake Erie rose
to as high as 5 m above datum. Evidence for higher lake levels include a “drowned
forest” at Clear Creek west of the Grand River outlet and undated, but contemporaneous, raised shorelines and deltas. The most probable cause for the higher lake
levels is abandonment of the glacial lake Nipissing II outlet channel northeast of
Lake Huron and passage of much greater water volumes from the upper to lower
Great Lakes. Under such conditions, an embayment of Lake Erie would have
formed (Coakley, 1992:Fig. 10) extending to, and possibly beyond, the Grand
Banks (km 35 from the outlet), thereby substantially reducing river gradient. This
would have induced aggradation of sediments and could have accounted for the
incipient development of the Grand Banks lateral bar at some time before 3900
B.P.
After 3900 B.P., lake-level trends are not fully resolved, but the preferred hy-
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pothesis is a rapid drop in lake levels to 5 m b.d. in response to widening of the
Niagara outlet, followed by a very gradual rise (10 cm/100 years) to modern datum.
A rapid drop in lake levels after 3900 B.P. would invoke some entrenchment and
make the existing bar surface accessible only to the largest floods. Under these
conditions, a surface A horizon (PI) would develop as pedogensis proceeded, coupled with a much reduced frequency of sediment input. As levels in Lake Erie rose,
the bar would aggrade gradually to accommodate the higher base level. As such,
an equilibrium height of the bar may have been achieved by 1500 – 1000 B.P. (14C
years) as Lake Erie stabilized to near modern levels. At equilibrium height, floodplain inundation would again become less frequent compared to the previous accretion phase, thereby allowing the upper soil to form.
Of some importance to this model is the Nipissing II diversion at 3900 B.P. Coakley and Lewis (1985) and Anderson and Lewis (1985) have argued for it consistently
in constructing Lake Ontario and Lake Erie Holocene lake levels. In contrast, Flint
et al. (1988) and Weninger and McAndrews (1989) found little evidence for it in
flood-ponds near basin outlets at the west end of Lake Ontario. The need to better
define water levels in the middle Holocene represents an area in need of further
research. Our assumption here is that the archaeological evidence from Grand
Banks and other outlet river valley sites of the lower Great Lakes will provide good
complementary evidence for the late Holocene water level history.
DISCUSSION
The sedimentological and geomorphic evidence suggest five distinct phases in
the development of the Grand Banks lateral bar.
Stage 1: Valley Entrenchment and Widening
The late glacial and early Holocene base levels in Lake Erie were significantly
lower than modern datum. High meltwater discharge from northward and eastward
retreating ice lobes would have promoted entrenchment and widening of the lower
Grand River valley. Early Holocene channel conditions are unknown, but there is
a high probability that the channel was split and entrenched into the glacial clay
diamict and bedrock, particularly in the area of Grand Banks. A phase of higher
Lake Erie water levels between 5000 and 4000 B.P. resulted in slackwater conditions at the site and deposition of silts above the basal clay unit.
Stage 2: Development of the Lower Buried Soil (PI)
A rapid drop in Lake Erie base level following the Nipissing II maximum produced a steeper river gradient. Vertical degradation of the channel proceeded upstream from the lake reaching Grand Banks shortly thereafter. Downcutting may
have been slow in this reach due to exposures of resistant carbonate bedrock in
the valley bottom. Upstream of the site, erosion-resistant materials of compact
glaciolacustrine clays and bedrock comprising the concave bank (outer bank on
the west side) caused deflection of the flow (see Figure 3). A shift in the channel
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thalweg towards the opposite convex bank created a region of expanded or divergent flow in the downstream section of the concave bank along the west valley
wall. Under these conditions sediments would have aggraded very slowly on the
surface of the Grand Banks bar. Sufficient time was then available for the development of PI. Sometime during the development of PI, people made their first
recognizable appearance at Grand Banks.
Stage 3: Bar Development and Aggradation
As the river adjusted to gradually increasing lake levels, a lower gradient, and
possibly an increase in sediment supply, the frequency of vertical accretion events
increased. This resulted in the gradual aggradation of the bar surface. It is probable
that during the early stages of this phase the bar remained isolated from the concave bank by a major back channel or chute that was active only during highmagnitude floods. Subsequently, progressive infilling of the secondary channel occurred. Movement of water and sediment into the back channel resulted in selective
erosion of the lower soil (PI) in a series of small bifurcating chute channels connecting the bar-head to the downstream sections of the bar-chute (Figure 5). Some
artifacts associated with PI were reworked and incorporated into the overlying
sediment.
Stage 4: Development of the Upper Buried Soil (PII)
Continued aggradation to at least 1500 B.P. built the bar to an equilibrium height
when relatively stable conditions prevailed, thereby allowing the upper soil (PII)
to develop. As the base level in Lake Erie gradually approached modern levels,
continuing stability of the bar surface (i.e., limited surface erosion and minimal
sediment input) would have required a reduction in the frequency of overbank
flooding and the amount of sediment deposited during each event. The second
human occupation of Grand Banks began at least by 1600 B.P. and lasted until
probably 1000 B.P.
Stage 5: Post (European) Settlement Alluvium (PSA)
The uniformly coarser silts capping the upper soil were deposited sometime after
A.D. 1000 primarily as vertically accreted sediment during overbank flooding. Other
studies have reported that preservation of soil horizons is facilitated by rapid and
episodic deposition of alluvial sediment atop the organic facies (Hayward and Fenwick, 1983; Kraus and Brown, 1986). The spatial variability in PSA thickness (Figure
8) suggests vertical accretion was more continuous or proceeded at higher rates
away from the zones of localized scour such as in the back-chute and bar-head
regions. Our data indicate a relationship between postsettlement alluvium (PSA)
thickness and preservation of PII (hence the Princess Point occupation surface)
such that PII is best preserved in regions of thickest PSA accumulation. Where the
PSA is less than 20 cm thick, plow disturbance may be responsible for the absence
of PII.
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The charcoal sample from the PSA suggests a contemporary age ranging from
A.D. 1650 to 1950. Increased sediment yield due to forest clearance and the introduction of European agriculture are known to produce thick floodplain deposits.
Magilligan (1985, 1992), for example, documented land use changes that produced
approximately 3 m of floodplain alluviation over the past 160 years in a catchment
of the upper midwestern United States. Similar occurrences in watersheds of southern Ontario have generally not been documented despite erosion studies which
point to agriculturally derived sediment yields being very high (Wall et al., 1982).
Willson (1993) found buried soils in the nearby Saugeen River basin, although these
did not exceed 20 cm in thickness. Both Willson (1993) and Campo and Desloges
(1994) attributed thin or absent PSA to the fine-grained nature of the source sediments (glaciolacustrine clays), which, when eroded, are conveyed directly to the
basin outlet as wash load. Similar fine-grained sediments dominate selected reaches
of the upper and middle Grand River.
The formation of the both the lower and upper soil suggests periods of long-term
stability of the floodplain surface prior to European settlement. Since two phases
of floodplain stability have been documented in other late-Holocene valley fills of
eastern North America that were not influenced by base level changes (Brackenridge, 1984), environmental effects other than base-level changes may be important.
For instance, a dryer (and warmer) climate might reduce overbank flood frequencies (Knox, 1993). Crawford et al. (in press) reviewed local and regional climate
reconstructions and found that for southern Ontario there is no clear climate signal
that might explain reduced river discharges prior to the 17th century. A reduction
in sediment supply would also limit the amount of overbank sedimentation. Our
initial assumption was that land clearance by Iroquois people might have actually
increased surface erosion contributing to burial of the upper soil. However, Campbell and Campbell (1994) estimate that prior to A.D. 1600 Iroquoian peoples probably disturbed (burned, cleared, or cultivated) no more than 3.2% of the Southern
Ontario landscape. It is unlikely this had sufficient impact on sediment yields prior
to European settlement. The major impact was European land settlement between
A.D. 1800 and 1900 when as much as much as 65% of the land area had been cleared
(Kelly, 1974).
CONCLUSIONS
Geoarchaeological research at Grand Banks is contributing to our understanding
of a complex developmental history of the site. In the prehistoric period, middle
Holocene hunter-gatherers and late Holocene horticulturalists utilized the Grand
Banks floodplain. Grand Banks is a lateral bar primarily formed by vertical accretion in a reach with very high lateral stability of the main channel. Vertical accretion
proceeded at an average rate of 5 – 7 cm/century over the last 3200 years. This is
comparable to vertical accretion rates observed elsewhere in Ontario (Stewart et
al., 1991; Willson, 1993), but total accretion is low because of the absence of a
laterally migrating channel. Low rates of sediment production and a high rate of
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conveyance of fine-grained sediment (wash load) to the basin outlet at Lake Erie
may also account for the low total. Initial bar formation was associated with slack
water conditions that were probably controlled by high water levels in Lake Erie.
The Grand River has been subject to very low rates of lateral migration throughout
the late Holocene. This has preserved artifacts of at least two cultural occupations.
Low stream power, coupled with cohesive bank materials (silt and clay), are the
most important factors restricting lateral migration. The lower Grand River floodplain has a high preservation potential for occupations of the last 1500 years and
up to the last 4000 years.
Human occupation coincided with the formation of the two buried soils. Radiocarbon dating of archaeological plant remains is helping to detail the chronology
of both surfaces. The lower soil contains evidence of a Late Archaic/Early Woodland occupation at around 3200 B.P. (14C years). This horizon has not yet been
explored in detail. The upper soil is a single unit with no vertically separated Princess Point horizons. Instead, evidence is for a 6th to 10th or 11th century A.D.
period of Princess Point use of the floodplain at Grand Banks. Episodes of occupation cannot yet be confirmed, but there is a possibility of horizontal separation
of activities having different ages. That is, Area A has three early 14C dates on corn
relative to the single late corn dates from areas B and C. Local human activity that
could have disrupted local habitats appears to have been small scale.
Preservation of old soils and cultural artifacts at Grand Banks is in areas of the
thickest PSA accumulation or those areas not subject to surface channeling and
subsequent sediment removal. At Grand Banks channeling is restricted to a zone
that connects the bar-head area to the back chute regions, and, thus, these environments are the least attractive for preservation of cultural materials. The back
chute was more active early on in the formation of the bar selectively removing
sediments and possibly evidence of PI occupation.
Stratigraphic and cultural remains both point to periods of late Holocene stability
of the Grand Banks site. Several factors probably contributed to the reduced flood
frequencies and/or lower sediment input during soil formation. Our investigation
indicates a probable high sensitivity, at least initially, to base level changes. There
appears to be no analogue in the regional pollen record to suggest that significant
climate change might explain later periods of stability. More detailed investigations
of upstream and downstream floodplain configurations as well as the examination
of the local pollen record will facilitate resolution of the problem.
The Grand Banks settlement location appears to have been on the highest elevation on the floodplain. Of course, archaeological recovery and visibility of the
Princess Point occupation may be enhanced at this location because it is generally
situated in a buried soil preserved by a thick PSA that protects the cultural horizon
from scouring and plowing. Nevertheless, the site where we have been excavating
would have afforded some protection from minor overbank flows upstream and
downstream from the site. More severe floods during this century have a probability
of 0.05 or recurrence interval of 20 years. This supports our contention made elsewhere that longer-term Princess Point settlements, rather than short-term seasonal
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camps, should not be ruled out at Grand Banks (Crawford et al., 1997b). The existence of two buried soils over much of the lateral bar also argues for a relatively
stable period during the Princess Point use of the floodplain. An earlier stable
period also facilitated hunter-gatherer use of the floodplain, but we have not explored this period of human use in any detail.
Little or no evidence of prehistoric occupation after A.D. 1100 indicates site
abandonment after the Princess Point period. Other Princess Point sites such as
Forster and Middleport also have later Glen Meyer occupations on them. These
sites are not situated on lower-elevation floodplain sites, however. The unique
Grand Banks setting may have become unfavorable for year-round occupation because of flooding associated with cooler and wetter conditions in eastern North
America after A.D. 1200 (Baron, 1992).
We would like to thank B. Kawecki for invaluable field and laboratory assistance. Additional field support
was graciously supplied by C. Shen, V. Bowyer, A. Hawkins, J. Quinn, and T. Ormerod. This research
was supported by the Social Science (SSHRC) and Natural Sciences and Engineering (NSERC) Research
Councils of Canada to G. Crawford, J. Desloges, and D. Smith. G. Running and an anonymous referee
carefully reviewed the manuscript.
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