Chapter 5 AW V.’.V Wjji V.W W.1.1 V-’v VAV switf.1.1.11 V V V V v • V V V V V V • V V V V V V V The Evolution of Landforms at Keatley Creek, near Lillooet, British Columbia Pierre Friele • • t t • A A A A A A A ■• •# •• «• •■_ Introduction Bedrock Geology The objectives of this chapter are to describe the The main bedrock mass underlying the Clear Range basic geological context of the Keatley Creek site and consists of fine grained sedimentary rocks (argillite) and to outline the evolution of landforms at the site. Under­ intercalated chert, including smaller outcrops of standing the nature of the geological matrix and pro­ limestone and volcanic rock (basalt and tuff). These cesses at archaeological sites is fundamental in docu­ rocks are middle to late Triassic in age and are part of menting and understanding site formation processes. the Cache Creek Group. This marine assemblage is This chapter involves 1) a brief description of the region­ intruded into by younger, plutonic rocks (granodiorite, al setting and bedrock geology; 2) a review of the local diorite) of Jurrassic age put into place during the Quaternary history, providing a conceptual model for formation of the Coast Mountains (Monger and the formation of the main landforms at the site; and 3) McMillan 1984). an airphoto analysis and field examination, providing Quaternary sediments underlie the study site and insight into post-glacial processes that have modified fill the Fraser Valley to an elevation of about 350 m. the main landforms and which may have affected the However, bedrock directly underlies the steep hillsides prehistoric inhabitants or contributed to the post­ to the east of the site. On the slopes on the south side of depositional modification of archaeological features. Keatley Creek are outcrops of dioritic rock; on the north side of the creek are volcaniclastic rocks (tuffs) Physiography containing minor basalt and andesite flows (Monger and McMillan 1984). At Keatley Creek, the Fraser River forms the boundary between the Camelsfoot Ranges to the northwest and the Clear Range to the southeast. These Late-Quaternary History ranges constitute the southwestern edge of the Fraser Surficial materials and landforms in the study area Plateau, a subdivision of the Interior Plateau (Holland are the product of Quaternary glaciation with minor 1976). This is rugged country, with steep slopes rising post-glacial modification (Ryder 1976). Thick glacial from about 250 m elevation along the deeply incised drift has filled the Fraser Valley to depths between 250­ Fraser River, to summit elevations in excess of 2,250 m 350 m. These sediments, consisting of sequences of in the Camelsfoot and Clear Ranges. glaciofluvial, glaciolacustrine and till materials, 65 Pierre Friele : Chapter 5 represent Early Wisconsinan and the Late Wisconsin an, In the vicinity of the Keatley Creek archaeological Fraser Glaciations (Huntley and Broster 1994). site, the Fraser River, graded to about the 250 m elevation, is incised to a depth of 330 m below the valley During the onset of the Fraser Glaciation (29,000­ fill terraces. The valley fill surface meets the bedrock 20,000 years BP), montane glaciers in the Camelsfoot hillslope at about the 640 m elevation. Above the terrace Range to the west built up and began to advance surface, bedrock-controlled slopes climb steeply into downvalley. In front of the ice, glaciofluvial outwash the Clear Range. The Glen Fraser and Keatley Creeks sediments filled the lower reaches of montane valleys flow from small, steep-sided basins which drain the and prograded into the Fraser Valley. Eventually, large west side of the Clear Range. These creeks are outwash fans, debris flow fans, and then tributary ice, intermittant and ephemeral, partially drying up during blocked southerly drainage of the Fraser River, forming the summer months. an extensive proglacial lake. This feature, extending from the study area, as far north as Williams Lake, has The valley fill terrace surface has two main levels: been called Glacial Lake Camelsfoot (Huntley and the upper terrace extends from the hillside at 640 m Broster 1994). As climate continued to deteriorate, elevation to a scarp at about 585 m elevation, about tributary glaciers coalesced in the main valleys and 500 m west of the hillside. The lower terrace is a gently eventually spilled onto Plateau areas to the east. During sloping feature with a scarp at 485 m elevation the climax stage (19,000-13,500 years BP), montane overlooking the Fraser River below. The upper terrace glaciation gave way to true piedmont glaciation, with surface is flat to undulating, and is underlain by about the direction of ice flow controlled by the rheology of 10-15 m of Fraser Glaciation till overlying advance an extensive ice dome rather than by topography. In glaciolacustrine silts. This sequence is exposed in the south central British Columbia, the location of the ice upper terrace scarp and in the steep gully sidewalls of divide was situated almost directly over the northern Glen Fraser Creek. The base of the lacustrine materials Camelsfoot Range. Following the climax stage, climate was not seen. The lower terrace is underlain by rapidly ameliorated, and deglaciation began. In coalescing, immediate postglacial, debris flow fans montane areas, ice thinned, exposing summits and which have issued from Glen Fraser, Keatley, and Sallus ridges, while valley glaciers gradually retreated into Creeks (Ryder 1976). Older drift materials are exposed the headwater areas. In the Interior, the piedmont dome in the steep scarps that descend directly to the Fraser stagnated, becoming widely disintegrated, locally River. Transverse to the orientation of the Fraser River blocking drainage, and causing the formation of large terrace scarps, are deep gullies incised by Glen Fraser, postglacial lakes. Deglaciation was largely complete by Keatley, and Sallus Creeks. Aeolian silts and sands, 11,000 years BP (Ryder et al. 1991). varying in thickness from .10-0.5 m, have capped the surface of the drift terrace. During and immediately following deglaciation, valley fill materials were rapidly incised, leaving broad The housepit site is situated on the upper terrace, drift terraces along the Fraser River. At the mouths of on the height of land between the Keatley and Glen steep tributary basins, large debris flow fans developed Fraser gullies. The main concentration of housepits is (Ryder 1971). Gradually, as sediment supplies declined, located in a southemly sloping swale. Given its sloping, these alluvial fans became incised, leaving the channel-like form defined by scarps to the east and landscape much as we see it today. Holocene processes west, this swale was probably carved by a transient, consist of minor fluvial reworking of drift materials, ice-lateral meltwater channel. The location of the minor debris flow activity, and aeolian reworking of housepits on the floor of the channel feature, a relatively fines scoured from terrace scarp faces (Ryder 1976). sheltered position, would have offered some protection from the strong, cold winds that blow down the Fraser Valley during the winter. The lower bedrock slopes, just above the drift Site Description and terrace, are blanketted with 2-4 m of till. This till blanket Formation Processes gradually thins upslope over a distance of 50-75 m Air photo interpretation (approximate scale Upper slopes consist of a mixture of thin rubbly 1:14,000) has been used to map the terrain features at colluvium, thin till, and exposed bedrock. Post-glacial the Keatley Creek site. Interpretations based on this hillslope processes, including slope wash and small analysis have been augmented with experience gained debris flow activity, have gullied the till blanket and from excavation and field examinations in the vicinity redeposited the material in small fans which spread of the site. The following discussion refers to specific out over the inner 50-100 m of the upper terrace. The landforms depicted in Figure 1. stratigraphy in HP 106 suggests that these processes 66 Evolution ofLandforms were active into the late Holocene, during the period slump, probably of early to mid-Holocene age, of housepit occupation. HP 106 was originally although possibly later. Presently, Glen Fraser Creek excavated into till or colluvial material, and then flows to the north side of this feature, along a relatively abandonned for some time allowing 10-15 cm of well shallow gully, before entering the main gully at the toe sorted aeolian silt and sand to accumulate on the rim. of the deposit. The creek arises from a spring near the Overlying the aeolian material is 20 cm of poorly sorted apex of the deposit. Examination of this area revealed sediment containing an angular volcanic clast about recent sediment trim lines 50 cm high on Douglas fir 25 cm in diameter. The angular clast is clearly derived tree trunks and a number of small, fresh debris lobes from the nearby hillslope, thus the layer may represent indicating active debris flow activity issuing from the a thin debris flow layer. Glen Fraser basin. These debris flows have the potential The large hummocky feature that occupies the to divert Glen Fraser Creek to the south, towards the mouth of Glen Fraser basin (Fig. 1) truncates the head housepit site. Topographic and stratigraphic evidence of the creek gully. This feature is interpreted as a large suggests that diversion has happened in the past. The southern tributary gully to the main Glen Fraser gully was prob­ ably carved when the creek flowed along the south side of the main lobe. The fine sediment which has filled HP 119, and adjacent pits in the west comer of the site, represents fluvially transported materials that have washed down from the south side of the Glen Fraser lobe.