Landscape evolution and human occupation during the Archaic period ...

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Landscape evolution and human occupation during the Archaic period on the northern Plains1 Gerald A. Oetelaar

Abstract: Recent archaeological excavations at a deeply stratified site in northwest Calgary, Alberta have uncovered four occupation layers spanning the interval from 8000 to 2000 BP. Depending on the dating one accepts for the Holocene thermal maximum, two or more of these occupations occurred during this interval of aridity. This site thus provides a record of human occupation during the Archaic (Middle Prehistoric I) period in the Calgary area. More importantly, the site occurs in an upland setting, apparently far removed from reliable sources of water and normal environments of deposition. Yet, the cultural deposits occur in sediments which extend 2 m below surface and include five sedimentary units as well as a minimum of three pedogenic units. This paper presents the results of geoarchaeological research on the changing landscape for the site and notes the implications of these changes for the study of Archaic manifestations on the northern Plains. Résumé : Des excavations archéologiques récentes à un site stratifié profond dans le nord-ouest de Calgary, Alberta, ont révélé quatre niveaux d’occupation qui couvrent l’intervalle de 8000 à 2000 ans avant le présent. Selon la datation que l’on accepte pour le maximum thermal à l’Holocène, deux ou plus des occupations ont eu lieu durant cet intervalle aride. Ce site fournit donc un registre de l’occupation humaine durant la période Archaïque (Préhistorique moyen 1) dans la région de Calgary. Plus important encore, le site se trouve sur un emplacement de hautes terres, apparemment loin de sources d’eau fiables et des environnements normaux de déposition. Toutefois, les dépôts culturels se retrouvent dans des sédiments qui s’étendent à deux mètres sous la surface et ils comprennent cinq unités sédimentaires et un minimum de trois unités pédogénétiques. Cet article présente les résultats de la recherche géo-archéologique sur le paysage changeant du site et note les implications de ces changements pour l’étude des manifestations de l’Archaïque dans les plaines du Nord. [Traduit par la Rédaction]

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Introduction Over two decades ago, Reeves (1973) argued that the lack of evidence for human occupation on the northern Plains during the Altithermal, a mid-Holocene episode of aridity, should be attributed to sampling bias, not the abandonment of the region. In his opinion, the perceived absence of a middle Holocene archaeological record in the area was the result of errors in typological classification and the widespread burial or erosion of sediments dating to this interval. Less than a decade later, Buchner (1980) suggested that human groups shifted their settlement strategies during this climatic episode, seeking refuge in river valleys and spring-fed upland locales with ready access to water and critical resources. In 1981, Wilson (1983) noted that the poor representation of sites dating to the PaleoIndian and Archaic periods in the Calgary area reflected primarily poor visibility, which he attributed to deep burial within the alluvial and colluvial deposits along the margins of the Bow River valley (see also Wilson 1990). Ten years later, Walker (1992) repeated Wilson’s conclusion

based on the discovery of deeply buried cultural deposits on a major terrace along the South Saskatchewan River in the Saskatoon area. The conclusions reached by these Canadian scholars, namely that geologic processes have probably removed the deposits from some middle Holocene sites and buried those of others, are echoed by most studies reported in the Archaeological Geology of the Archaic Period in North America (Bettis 1995). Yet, despite the accumulating evidence, dissenting voices persist, arguing instead for a substantive reduction in human occupation of the northern Plains during the Holocene thermal maximum (e.g., Forbis 1992; Husted 2002). In part influenced by the concept of refugia, geoarchaeologists studying the evolution of landforms on the northern Plains during the middle Holocene have focussed on the analyses of alluvial deposits within major river valleys and their tributaries (e.g., Albanese 2000; Artz 2000; Mandel 1992; Waters and Kuehn 1996). These studies generally include only limited references to the effect of geological processes on archaeological sites located in upland settings

Received 16 April 2003. Accepted 11 February 2004. Published on the NRC Research Press Web site at http://[email protected] on 21 June 2004. Paper handled by Associate Editor J.R. Desloges. G.A. Oetelaar. Department of Archaeology, University of Calgary, Calgary, AB T2N 1N4, Canada. (e-mail: [email protected]). 1

This article is one of a selection of papers published in this Special Issue on Applications of earth science techniques to archaeological problems.

Can. J. Earth Sci. 41: 725–740 (2004)

doi: 10.1139/E04-015

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somewhat removed from rivers. During the past two decades, however, several sites with deeply buried cultural deposits have been discovered in upland settings throughout southern Alberta (e.g., Van Dyke and Stewart 1985; Vivian et al. 1998). This improved discovery rate can be attributed to the implementation of deep testing as part of the initial Historical Resources Impact Assessments. The majority of the upland sites with deeply buried cultural layers tend to occur in depositional environments, namely in and around sedimentfilled depressions on hummocky disintegration moraines that are common around Calgary. Similar “non-traditional” geomorphological contexts have yielded valuable paleoecological data for the late Pleistocene and early Holocene throughout southern Alberta and Saskatchewan (e.g., Klassen 1994; Yansa 1998; Yansa and Basinger 1999). With some notable exceptions, however, few of the archaeological sites have been the subject of detailed geoarchaeological investigations. In an attempt to address this discrepancy, this paper presents the results of such research at one upland site in southern Alberta.

Study site Situated in northwest Calgary, the Tuscany site (EgPn-377) is located on a Pleistocene beach terrace (Wilson 1983, p. 165) at the confluence of the Bow River and Twelve Mile Coulée (Fig. 1). The site occurs in a bowl-like depression at an elevation of 1140 m above sea level or - 55 m above the level of the Bow River, now located 1 km to the south. A minimum of four occupation layers dating from 8000 to 2000 BP have been identified in sediments extending some 2 m below surface. Two or possibly three of these cultural occupations can be assigned to the Holocene thermal maximum or Hypsithermal. Across Canada, the dates for the onset and termination of this climatic interval are time transgressive but, in Alberta, the Hypsithermal is generally assumed to have lasted from 9200 to 5900 BP or slightly later (Anderson et al. 1989; Vance et al. 1995). On the northern Plains, the Hypsithermal is generally characterized as an interval of decreased precipitation and increased temperatures resulting in lowered water tables, increased salinity in ponds and lakes, expanded grasslands, and increased fire frequency (Anderson et al. 1989). Despite the inferred aridity of the Hypsithermal, human groups still chose to occupy this upland location, even though the nearest source of reliable water was almost 1 km away. More importantly, the resultant cultural deposits were preserved beneath several metres of sediment. To understand the nature and preservation of such cultural deposits, it is necessary to reconstruct the evolution of the local landscape, vegetation, and animal populations from the Pleistocene to the present. Although the reconstruction outlined below provides a specific context for interpreting the cultural deposits at this site, the results have implications for future research on Archaic adaptations on the northern Plains.

Field methods The reconstruction presented in this paper is based on the analyses of sediments, buried soils, and cultural deposits examined during three years of excavation at the Tuscany site. Between 1995 and 1997, a total of 90 contiguous 1 m ×

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1 m units were excavated near the centre of the depression containing the cultural deposits. By the end of the third field season, the main excavation attained maximum dimensions of 13 m north–south by 12 m east–west and a total depth of 2 m below surface. At this depth, essentially sterile sand was encountered and this stratum served to define the maximum depth of the archaeological excavations. To determine the horizontal extent of the cultural deposits, a series of 1 m × 2 m test units was dug at 10-m intervals north, south, east, and west of the main excavation (Fig. 2). Each unit was excavated down to the culturally sterile sand and thus revealed the horizontal extent of the various strata and pedogenic units identified in the main excavation. For ease of presentation, the test units are designated as E10, E20, and E30, where the letter represents the cardinal direction and the number refers to the distance from the main excavation. During the 1995 field season, a 1 m × 2 m exploratory trench was excavated through the basal sand in the main excavation and exposed a massive clay unit 3 m below surface. To understand the nature and extent of the sand layer, a series of six backhoe trenches was excavated during the 1997 field season. Four of the six trenches were aligned along the axes defined by the test units and were positioned - 40 m north, south, east, and west of the main excavation (Fig. 2). Of the remaining trenches, one was located immediately northwest of the main excavation, whereas the second was situated near the centre of a sand pit located approximately 100 m west of the site. Again, for ease of presentation, the backhoe excavations are referred to as the north, south, east, west, central, and sand-pit trenches (Fig. 2). A minimum of one wall in each excavation was cleaned, photographed, interpreted, and drawn to scale to facilitate detailed correlations of individual strata in the field and in the laboratory. Further, the profiles exposed in the walls of the excavations were examined at various times throughout the three field seasons by geoarchaeologists, geologists, geographers, and soil scientists from the University of Calgary, the University of Alberta, and the University of Lethbridge. In addition, students enrolled in the archaeological field school collected soil samples from the different strata and performed particle size analyses under the supervision of qualified personnel in the Department of Geography at the University of Calgary. As a result, the descriptions and interpretations presented are those of the author but they actually represent a synthesis of perspectives and opinions expressed by colleagues who may or may not agree with some or all of the information summarized in the following sections. During the archaeological investigations, 10 cm × 10 cm bulk soil samples were collected from the southwest corner of each 5- or 10-cm level excavated. These samples were processed in the laboratory using a water separation device (e.g., Pearsall 1989) to retrieve representative samples of the small organic and inorganic remains present in the cultural and natural strata at the site. Over 1000 soil samples were collected and processed, but only those located beneath the Mazama ash have been examined to date (Siegfried 2002). Samples of organic material from the archaeological components were also submitted for radiocarbon dating and, together with the Mazama ash, provide the chronological framework © 2004 NRC Canada

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Fig. 1. Map of the Bow River Valley showing the location of the site and the stratigraphic sections with Mazama ash and the early Holocene paleosol.

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Fig. 2. Map of the site showing the location of the main excavation, test units, and backhoe trenches. The sand-pit trench is located beyond the limits of the map. The horizontal extent of the tephra and the paleosols is also indicated.

for the environmental reconstructions presented later in the text.

Results The stratigraphic profile exposed in the main excavation at the site included five strata and a minimum of three pedogenic units. From oldest to youngest, the sedimentary units consisted of a lacustrine clay, a lacustrine sand, a palustrine silty sand, an aeolian silt and fine sand, and a volcanic ash (Fig. 3). In Fig 3, the contacts between the strata are represented as solid lines. Within the palustrine and aeolian sediments, there were two buried paleosols and a surface soil (Fig. 3). The horizon boundaries of these pedogenic units are indicated by dotted lines in Fig. 3. Each of these sedimentary and pedogenic units is described below in an attempt to identify changes in the depositional environments which contributed to the landform present at the site today.

The Lacustrine Clay At the base of the profile, - 3 m below surface, there was a thick, massive clay unit forming part of the Calgary Silts and Clays (Moran 1986; Wilson 1983). This late Pleistocene sediment, which has been traced laterally by numerous scholars on the basis of stratigraphic sections and borehole data (e.g., Moran 1986; Tharin 1960; Wilson 1983), constitutes the typical surface deposit in this area of the city. Although undated, the Calgary Silts and Clays are generally interpreted as sediments that accumulated at the bottom of glacial Lake Calgary sometime between 21 000 and 12 000 BP (Fisher 1999). The Lacustrine Sand At the bottom of the main excavation, a sand layer approximately 1 m thick rested unconformably upon the basal clay. Although initially described as a beach sand (Oetelaar et al. 1996), this sediment was later determined to have been reworked during the creation of an erosional channel (Oetelaar © 2004 NRC Canada

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Fig. 3. Photograph and profile drawing of the south wall in the main excavation showing the stratigraphic units and the important buried soils.

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and Zaychuk 1997). However, undisturbed beach deposits were exposed in the east, south, and sand-pit trenches where a basal layer of medium to coarse sand was overlain by a mantle of wind-blown silt and fine sand 50 to 80 cm thick. The sand layer extended from the base of the wind blown sediment to the bottom of the trenches some 3 m below surface. Where exposed, the sand exhibited planar bedding with occasional sloping cross-beds indicative of a beach environment. Significantly, the beds dipped slightly to the southeast, a direction which is consistent with the morphology of the glacial Lake Calgary basin. In the central trench, the bedded sand stratum rested unconformably on top of a massive clay unit. Rip-up clasts present in the lower portion of the sand unit again suggest an association with a beach environment (Fig. 4). In the west trench, a massive clay unit, - 20 cm thick, was present within the sand stratum approximately 1.5 m below surface (Fig. 4). Significantly, the sand units above and below this clay stratum exhibited planar bedding comparable to that exposed in the trenches discussed above. In the north trench, the bedded sand stratum was encountered at the bottom of the excavation beneath a similar massive clay unit. In this trench, however, the clay unit had slumped along a number of fault lines and was overlain by finely cross-bedded silt and sand (Fig. 4). The cross-bedding indicates that stream flow was from the north-northwest to the south-southeast. Given the absence of datable materials and the lack of direct stratigraphic correlation, the relationships among the sedimentary units exposed in the trenches are difficult to establish conclusively. The elevation, structure, and thickness of the thin clay unit exposed in the north and west trenches argue for continuity in this stratum. By contrast, the absence of this stratigraphic unit in the east, south, central, and sand-pit trenches suggests a facies change somewhere between the main excavation and the north trench. In fact, this facies change can be seen in the north trench where segments of the clay unit have slumped as a result of undercutting by flowing water. Therefore, the sedimentary units exposed in the west trench are interpreted as essentially undisturbed Calgary Silts and Clays where massive clay beds are interbedded with sand layers (Tharin 1960). In the north trench, the clay bed and lower sand stratum are undisturbed, but the sediments overlying the clay unit were redeposited by a stream as cross-bedded silt and sand. Finally, the strata exposed in the east, south, central, and sand-pit trenches represent beach sands associated with a later stage of glacial Lake Calgary (Moran 1986). Together, these sediments suggest the presence of both fluvial and lacustrine environments in the immediate vicinity of the site sometime after the retreat of the glaciers and the initial drainage of glacial Lake Calgary. However, the sediments exposed in the backhoe trenches may relate to one of the final stages of glacial Lake Calgary identified by Moran (1986, figs. 24, 26). At this time, the western limit of the lake occurred in the vicinity of the site with one or more streams flowing into the lake from the north or northwest (Fig. 1). The stream could represent the precursor to Twelve Mile Coulée and (or) some other water course originating to the north or northwest.


Erosional channel Unlike the lacustrine sediments described above, the sand exposed at the bottom of the main excavation contained a thin, yet distinct, gravel lens immediately above the basal clay (Fig. 3). The gravel lens was traced laterally through the eastern, western, and southern test units as well as the central trench (Figs. 5–7). Interpreted as a lag deposit, this gravel layer outlined a V-shaped channel oriented roughly northeast–southwest with a relatively steep gradient toward the northeast. The lower 50 cm of this erosional feature was filled in with reworked beach sand and capped with several thin clay lenses. The thin clay unit contained numerous shells of aquatic gastropods, identified as members of the genus Stagnicola (L.V. Hills, personal communication, 1997). Another 50 cm of sand separated the thin clay layers from the floor of the main excavation, which also sloped toward the northeast (Siegfried 2002, p. 368). Although there is no evidence of a fining-upward sequence, the sediments and gastropods indicate the occasional presence of a pond or slow moving water in the bottom of the erosional channel. The erosional channel, which formed sometime after the final draining of glacial Lake Calgary, was apparently created by runoff flowing to the northeast or toward Twelve Mile Coulée. The obvious change in the drainage pattern from the preceding interval probably reflects the establishment of the coulée and the diversion of local runoff toward this new channel. However, the volume of runoff appears to have been limited as indicated by the thickness of the lag deposit, the presence of clay layers, and the accumulation of sand in the channel. Thus, the erosional feature apparently started to fill in soon after its formation thereby creating a somewhat linear depression on the surface which then started to support vegetation. The Palustrine Sediments The sediments overlying the basal sand in the main excavation graded from a fine sandy silt to a clayey silt and were characterized by increased concentrations of organic matter. This stratigraphic unit was of limited horizontal extent since it was not encountered in any of the peripheral test units or the adjoining backhoe trenches. In the northeast corner of the main excavation, the stratum was over 75 cm thick, while in the southwest corner it was only 35 cm thick. The variation in the thickness of this sedimentary unit is consistent with the topography of the erosional feature and indicates a gradual filling of the depression. A dark, reddish-brown (5YR3/2) band present near the bottom of the unit yielded carbonate concretions often associated with oxidized pond sediments in marshy environments. Given the limited horizontal extent of the deposit and its association with an erosional channel, this sedimentary unit appears to have accumulated in a palustrine environment — an inference that is supported by the assemblage of terrestrial gastropods recovered from the associated bulk soil samples (Oetelaar and Zaychuk 1997). The presence of a palustrine environment within the erosional channel is also indicated by the botanical remains recovered from the bulk soil samples (Siegfried 2002). A sample of charred plant remains from just above the contact between the palustrine sediment and the basal sand was submitted to Isotrace Laboratory and yielded a date of © 2004 NRC Canada

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Fig. 4. Profiles and photographs of the central, west, and north trenches showing the silt and fine sand, the medium to coarse sand, and the clay units.

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Fig. 5. Profiles of the eastern test units showing the gravel layer, the lower paleosol, and the Mazama ash.


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Fig. 6. Profiles of the western test units showing the gravel layer, the lower paleosol, and the Mazama ash.

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Fig. 7. Profiles of the southern test units showing the gravel layer, the lower paleosol, and the Mazama ash.


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735 Table 1. A list of uncalibrated conventional radiocarbon dates in years before present (BP) obtained on materials recovered from the section exposed in the main excavation at the Tuscany site (EgPn-377). Site

Level

Description

Age (years)

Lab No.

EgPn-377 EgPn-377 EgPn-377 EgPn-377 EgPn-377 EgPn-377 EgPn-377

upper paleosol upper paleosol lower paleosol lower paleosol lower paleosol hearth in lower paleosol base of palustrine stratum

bison left metatarsal bison third phalanx bison right metacarpal bison right femur (same bone) bison right femur charred plant fragments charred plant fragments

6775±40 6940±70 7310±45 7475±45 7610±70 7840±100 9950±120

OxA-11585 TO-9261 OxA-11583 OxA-11622 TO-9264 TO-9263 TO-9262

9950 ± 120 years BP (Table 1). By 10 000 BP then, the former erosional channel had filled in and was little more than a moist depression that gradually accumulated sediment and supported a plant community. The clastic sediments that accumulated in this linear depression were probably introduced by wind and, to a lesser extent, water and gravity. A well-defined buried soil with a thin, black (5YR2.5/1) Ah horizon and a distinctive dark reddish-brown (5YR2.5/2) B horizon was clearly identifiable near the top of the palustrine sediments. Although absent in the backhoe trenches and the northern test units, this paleosol was traced laterally for distances of 20–30 m east, west, and south of the main excavation, where it developed in aeolian silt and sand (Fig. 2). Like the gravel lens and the base of the main excavation, the paleosol outlined a depression that sloped to the northeast. By this time then, the outline of the channel was still identifiable but the relief of the depression had decreased somewhat. Since the soil developed not only in the palustrine sediments but also in the nearby aeolian silt and sand, the associated episode of landscape stability must have influenced more than the local depression with its higher moisture content. In fact, early Holocene paleosols are relatively common in southern Alberta (Valentine et al. 1987; Waters and Rutter 1984), occurring at a minimum of six stratigraphic sections in and around the city of Calgary (Fig. 1; Table 2). As noted in Table 2, the paleosols have been classified using a variety of names reflecting not only the naming preferences of individual researchers but also a degree of spatial variability in the soils. This spatial variability is also evident in soil chemistry and the associated assemblages of phytoliths and gastropods, which suggest moist environments with interspersed patches of woodland and grassland (Waters and Rutter 1984). Since the paleosols occur in both floodplain environments and upland settings well beyond the inferred limits of seasonal flooding, the increased moisture cannot be attributed solely to the local setting at the time of soil formation. Instead, the data suggest that the episode of landscape stability is associated with an interval of increased moisture availability, a conclusion that is supported by the detailed analysis of the 8000-year-old Eutric Brunisol in the Livingston Gap (Reeves and Dormaar 1972). At the Tuscany site, the earliest cultural occupation was associated with this distinctive paleosol. The assemblage of artifacts recovered from this occupation included flaked stone tools, debitage, fire-broken rock, and bone fragments. The nature of the assemblage, as well as the distribution of the materials, is consistent with an occupation by at least one

household for an undetermined amount of time. Charred plant remains from a hearth and nearby bone fragments were submitted to Isotrace and Oxford Laboratories yielding dates ranging from 7310 to 7840 BP (Table 1). These dates are somewhat younger than those obtained from other paleosols (Table 2), although the slight differences can be attributed to the nature of the material being dated. The charred plant remains from the hearth, for example, yielded an older date than the bone samples from the same level (Table 1). In the reconstruction of the vegetation on the site at the time of this occupation, Siegfried (2002) argues for a parkland setting with white spruce, willow or poplar, kinnikinnik, and prickly rose. This reconstruction is corroborated by the assemblage of terrestrial gastropods and mammal remains, which includes species or genera commonly found in wooded and grassland environments (Oetelaar et al. 1996). Together then, the data from the site indicate an episode of landscape stability sometime between 7500 and 8000 BP. The stable landscape appears to coincide with an increase in available moisture and, perhaps, a decrease in temperature. Further, this stable land surface supported a floral community reminiscent of the modern parkland with an open spruce and poplar forest. Indigenous groups moving through this landscape probably selected this upland location as a suitable campsite because it offered expansive views of the floodplain, as well as ready access to the Bow River via Twelve Mile Coulée. The Aeolian Sediments The upper 1.5 m of the profile exposed at the Tuscany site consisted of aeolian sediments, primarily silt and very fine sand. A tephra, encountered 1–1.4 m below surface, serves to subdivide the aeolian sediment into three units, each of which is described below. Pre-Mazama deposits Approximately 10–15 cm of aeolian sand, including a thin buried soil, separated the top of the palustrine sediment and the Mazama ash. The buried soil, located immediately below the Mazama ash, included a clearly defined, very dark brown (10YR2/2) Ah horizon overlying a calcium enriched C horizon. This paleosol was identifiable only in the main excavation where sediment accumulation was apparently greatest (Fig. 2). The absence of terrestrial gastropods in the flotation samples from this paleosol suggests the presence of a drier environment, an inference that is corroborated by the assemblage of botanical remains, which is dominated by grasses and forbs (Siegfried 2002). Paleosols in similar stratigraphic contexts have been identified throughout the province (e.g., Kingston © 2004 NRC Canada

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Table 2. Names, descriptive data, dates and sources for stratigraphic sections with early Holocene paleosols in southern Alberta. Distance below ash (cm)

Localitya

Name

1 2 3 4

Harvey Bearspaw EgPn-428 Bowfort

5 6

Highway 1 Mona Lisa

20 45 0–20 87 75 32.5 45–78

Fish Creek

0–80

Priddis 6

Joffre Bridge

2–14

Willow Creek

14

Livingston Gap

45

Summit Lake

0

Lethbridge

60

Source

Cumulic Humic Regosol Cumulic Humic Regosol Chernozem

8400±150 years BP (GSC-1819) 8710±230 years BP (RL-905) 8080±150 years BP (GSC-1209) 8300±280 years BP (GX-6397-A)

Rego Dark Brown Chernozem; Orthic Dark Brown Chernozem Solonetzic Black Chernozem Cumulic Humic Regosol Gleyed Dark Brown Chernozem Eluviated Black Chernozem Degrded Alpine Calcareous Black Chernozem Cumulic Humic Regosol

See Fig. 1 for locations of the numbered sections. This date derives from aquatic gastropods. c This date was considered less reliable than GSC-1158. b

Date

Oetelaar 2002 Waters 1979; Waters and Rutter 1984 Vivian et al. 1998 Harrison 1973; Wilson 1983 Waters and Rutter 1984 Wilson 1974, 1983 Wilson 1974, 1983 Dormaar 1983; Kingston 1982 Waters and Rutter 1984 Sanborn and Pawluk 1980

8030±200 years BP (GSC-1944)

9290±260 years BP (GSC-236)

b

8050±150 years BP (GSC-1158) Eutric Brunisol

Dormaar 1983; Lowton et al. 1974; Waters 1979; Waters and Rutter 1984 Waters 1979; Waters and Rutter 1984 Dormaar 1983; Dyke et al. 1965; Waters 1979; Waters and Rutter 1984 Dormaar 1976, 1983 9570±240 years BP (GX-0956) c Waters 1979; Waters and Rutter 1984 Waters 1979; Waters and Rutter 1984

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a

Rough Creek

Soil type

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1982; Reeves and Dormaar 1972; Waters and Rutter 1984; Wilson 1983) and these occur in both alluvial and upland settings. Where and when analysed, these buried soils are normally associated with grassland environments. The second cultural component identified at the site was associated with this paleosol. The assemblage of artifacts included one projectile point, a few flakes, and the remains of bison. The low density and diversity in the assemblage of artifacts indicates the presence of a small kill site. That is, people appear to have camped at this location for a very brief interval, perhaps after a successful hunting episode. As before, the occupation occurred during an episode of landscape stability that one associated with the establishment of a grassland environment sometime around 6800 BP (Table 1) or shortly before the eruption of Mount Mazama 6730 BP (Hallett et al. 1997; Zdanowich et al. 1999). Mazama ash At Tuscany, the tephra layer occurred 1–1.4 m below surface and extended only 20 m east, south, and west of the main excavation (Fig. 2). Its absence in the peripheral units and trenches, however, can be attributed to the effects of erosion, probably by wind. Beyond the site, Mazama ash has been noted in terraces along the Bow River immediately south of the site, within abandoned channels and ponds along Twelve Mile Coulée, in bluff edge dunes along the valley margins, and in depressions on the neighbouring uplands. Using this chronostratigraphic marker, it is possible to reconstruct the broader landscape sometime around 6730 BP. At this time, the Bow River was flowing in an entrenched valley some 45 m below the site, Twelve Mile Coulée was well established east of the site, the bluff edge dune was starting to form, and the depressions on the neighbouring uplands were somewhat deeper than they are today. Surface sediments Above the ash, there was a thick (ca. 1.2 m), massive aeolian unit with a cumulic surface soil. As for the lower aeolian silt and sand, these sediments accumulated as a bluff edge dune (e.g., David 1970) and transformed the linear channel into the bowl-like depression observed on the surface today. Although the surface sediments span the last 6730 years, the accumulation rate of the silt and fine sand appears to have been variable. The absence of buried soils in the lower (ca. 80 cm) portion of this unit suggests that the accumulation rate after the deposition of the tephra may have been sufficiently rapid to preclude soil formation. Sometime around 5000 BP, as indicated by the diagnostic artifacts (i.e., Oxbow points), the rate of sediment accumulation appears to have decreased sufficiently to allow the establishment of vegetation and the formation of the surface soil. The third distinct cultural component occurred - 55 cm below surface near the base of the cumulic Ah horizon. This occupation was represented by a partial tipi ring and a relatively dense scatter of tools, debitage, fire-broken rock, and bone fragments. With the exception of a few canid bones, the highly fragmented faunal remains derive primarily from bison. The assemblage of artifacts, which includes several Oxbow points, suggests a seasonal encampment sometime around 5000 BP. Since the Oxbow occupation, - 55 cm of sediment has

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accumulated in the depression but, apparently, the rate of accumulation has never been sufficient to impede plant growth as indicated by the cumulic Ah horizon of the surface soil. The fourth cultural occupation is represented by a very diffuse scatter of cultural material within the top 25 cm of the profile. Typologically, this material is younger than 2000 BP and thus postdates the interval of interest in this study.

Discussion Several models of landscape changes in southern Alberta have been proposed in recent years but only two are relevant here. According to Campbell and Campbell (1997), the important geomorphic events during the Late Pleistocene and Holocene relate to variations in the seasonality of insolation produced by changes in the Earth’s orbital elements. After correcting all dates for 13C fractionation and calibrating existing radiocarbon dates from southern Alberta, they subdivide the late glacial and postglacial periods into four major episodes, each dominated by a particular set of geomorphic processes. The four episodes include (1) glaciation (>20 000 calibrated (cal.) years BP), when erosion and deposition by ice and subglacial meltwater predominated; (2) deglaciation (ca. 20 000 – 12 000 cal. years BP), when erosion by water was dominant; (3) maximum landscape stability (ca. 12 000 – 10 000 cal. years BP), when deposition was minimal, and (4) landscape instability (ca. 10 000 – 0 cal. years BP), when alternating phases of channel degradation and aggradation and dune activation and stabilization were very common. Sauchyn (1993a, 1993b) also sees climate as the primary forcing mechanism of landscape change during the Late Pleistocene and Holocene. His model of landscape change complements that of Campbell and Campbell (1997) but offers slightly greater resolution for the Holocene interval. He subdivides the Late Pleistocene and Holocene into four episodes, including (1) a Late Pleistocene episode of deglaciation, often accompanied by mass wasting and paraglacial activity; (2) a brief interval of early Holocene landscape stability (ca. 10 000 – 8000 BP); (3) a period of accelerated aeolian and fluvial erosion during the Hypsithermal (ca. 7900–5000 BP); and (4) a late Holocene episode of soil formation on relatively stable landscapes where only slopes and channels adjust to increased effective moisture (ca. 5000 BP to present). Both models identify prolonged episodes of landscape instability separated by a brief interval of landscape stability during the late Pleistocene and early Holocene. The landscape reconstruction for the Tuscany site is certainly consistent with such regional geomorphological changes. At the site, the redeposited beach sands and the erosional channel responsible for the depression presently occupied by the site are representative of the Late Pleistocene interval of unstable landscapes. The early Holocene episode of landscape stability, in turn, is represented by the palustrine sediments, including the buried soil. At the Tuscany site, this interval appears to have lasted slightly longer, from - 10 000 to 7500 BP. For the mid-Holocene, the episode of increased aeolian and fluvial erosion identified by Sauchyn (1993a) is represented by the aeolian sediments extending from beneath the Mazama ash to the bottom of the surface soil. The presence of a buried soil immediately below Mazama ash, however, suggests that the interval of greatest instability in this area occurred slightly © 2004 NRC Canada

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later or after the deposition of the tephra. Finally, the interval of late Holocene landscape stability identified by Sauchyn (1993a) appears to correlate with the cumulic Ah horizon of the surface soil. The reconstruction of the local vegetation based on the assemblages of botanical and faunal remains can also be compared to regional patterns of vegetation change based on pollen cores. Although several reconstructions of postglacial vegetation changes on the northern Plains are available (Vance 1986), the record from Toboggan Lake provides the best framework for interpreting Holocene paleoenvironments in the Calgary area (Beaudoin 1993). At Toboggan Lake, MacDonald (1989) identifies an early (ca. 10 200) grassland or parkland community that is replaced by an open spruce forest about 9400 BP. One thousand years later, the open spruce forest changes to a parkland environment which is gradually replaced by grassland about 7600 BP. The grassland community persists until 5500 BP at which time the modern spruce and pine forest becomes established. The record of vegetation changes inferred from the Toboggan Lake core is represented in the assemblages collected from the Tuscany site and provides an important link between the terrestrial and lake records for the area. For example, the assemblage of gastropods and botanical remains recovered from the palustrine sediments, dated between 9900 and 7600 BP, is consistent with an open spruce forest and parkland environment. Further, the nature of the buried soil and the assemblage of faunal remains recovered from the associated cultural deposit corroborate this inference. Similarly, the assemblage of plant remains recovered from the buried soil located immediately below Mazama ash is consistent with the establishment of a grassland in the area sometime around 7600 BP. Again, the assemblage of cultural materials, especially the butchered animal remains, associated with this paleosol corroborate this reconstruction. Finally, the cumulic soil, the buried tipi ring, and the associated faunal remains indicate the establishment and persistence of modern grasslands on stable landscapes from about 5000 BP to the present.


ous models of landscape evolution in southern Alberta and with the vegetation changes recorded at Toboggan Lake. At the Tuscany site, the evidence of human occupation is associated with the episodes of landscape stability. That is, the initial human occupation occurred when this upland location consisted of a moist linear depression supporting trees and shrubs. Human groups may have found this spot appealing as a campsite because the upland location offered expansive views of the nearby floodplain, as well as ready access to the Bow River via Twelve Mile Coulée. Further, water and other critical resources were probably readily available in and around the coulée and the numerous depressions on the nearby uplands. About 1000 years later, or just before the eruption of Mount Mazama, this locality was used by a second group of people who seem to have killed and butchered some bison. This group, however, did not camp at this location for any length of time. During this occupation, the relief of the landscape had probably changed very little although the vegetation was now dominated by grasses. Following the deposition of the Mazama ash, human groups did not return to set up camp at this locality until 5000 BP when grassland vegetation stabilized the sediments on the bluff edge dune. Geoarchaeological research at the Tuscany site clearly indicates that archaeological deposits dating to the late Paleoindian and early Archaic periods on the northern Plains stand a good chance of being preserved not only in river valleys but also in upland settings. Where preserved, the cultural materials will most likely be buried beneath several metres of sediments and will be associated with the early Holocene episode of landscape stability. The presence of a stable landscape and a moist parkland environment in this area - 8000 BP has important implications for our assessments of the timing and severity of the early Holocene thermal maximum on the northern Plains. At the same time, the widespread distribution of the Mazama ash and the early Holocene paleosol across southern Alberta offers archaeologists an important set of markers to help identify suitable locations for evidence of human occupation during the Archaic period.

Conclusion In summary then, the sediments, buried soils, and cultural deposits uncovered at the Tuscany site have enabled us to monitor the evolution of the local landscape and to reconstruct the changes in the local plant and animal communities. Based on the results of this study, the local landscape was relatively unstable during the late Pleistocene and early Holocene, or from the time of glacial lake drainage until the establishment of vegetation sometime around 10 000 BP. Thereafter, the landscape was stable for approximately 3000 years, during which time an open forest or parkland community became established in the area. Sometime around 7500 BP, the parkland vegetation was replaced by grasses, perhaps as a result of a moisture deficit associated with the Hypsithermal. The episode of maximum aridity and landscape instability occurred between the eruption of Mount Mazama, about 6730 BP, and the formation of the surface soil sometime around 5000 BP. Thereafter, the area again became stable and supported a grassland community. This reconstruction of the local landscape and vegetation is consistent with previ-

Acknowledgements Many of the ideas presented here derive from research funded by Carma Developers Ltd. of Calgary, Alberta and the Social Sciences and Humanities Research Council (SSHRC) through its Major Collaborative Research Initiatives (MCRI) grant (412-1999-1000). In addition, the radiocarbon dates from the Oxford Laboratory have been funded through a grant awarded to Dr. Alan Cooper. This paper has benefitted greatly from discussions with Alwynne Beaudoin, Andrea Freeman, John Dormaar, Len Hills, Brian Moorman, Gerald Osborn, Brian Reeves, Arlene Rosen, Darren Sjogren, and Derald Smith, all of whom spent some time examining and discussing the profiles with me. The constructive comments of the anonymous reviewers and the Associate Editor, Joe Desloges, have greatly improved the manuscript. Gerald Newlands photographed the profiles, while Amanda Dow, Stacey Zaychuk, and Dominique Cossu prepared the illustrations. Of course, I alone am responsible for any errors or omissions. © 2004 NRC Canada

Oetelaar

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