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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. CCC 0883-6353/97/080865-23

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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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FLOODPLAIN FORMATION, LOWER GRAND RIVER, ONTARIO

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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Received November 15, 1996 Accepted for publication June 9, 1997

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