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The Cowichan Head Formation and Dashwood Till (part of the Dashwood Drift geological unit) form a single aquitard. Cowichan Head Formation unconformably overlies the Dashwood Drift and is up to 21 m thick (Hicock and Armstrong, 1983). The formation has been divided into a lower member of clayey silt and sand with marine shells, and an upper member of sandy silt and gravel, commonly with reddish oxidized hues rich in fossil plant remains (Armstrong and Clague, 1977). There is at least 2.4 m of compact peat, probably deposited in a boggy floodplain (Alley, 1979). Radiocarbon dates range from 25.8 to 40.5 ka 14C BP (Armstrong and Clague, 1977). Fossil pollen and beetle assemblages suggest that the climate fluctuated between conditions similar to present and cooler than present (Armstrong and Clague, 1977). Gravel provenance is a mix of volcanic and sedimentary rocks (Armstrong and Clague, 1977; Clague, 1976). Geochemically the Cowichan has metal concentration similar to Dashwood. The lower Member is interpreted as glaciomarine, whereas the upper Member is attributed to estuarine and fluvial environments (Armstrong and Clague, 1977).
Dashwood Drift conformably overlies the Mapleguard Sediments and is commonly less than 10 m thick (Armstrong and Clague, 1977). The section of Dashwood Drift, which is comprised in the Cowichan-Dashwood hydrogeological unit, is composed of muddy diamicton and glaciomarine sediments (Armstrong and Clague, 1977; Fyles, 1963; Hicock and Armstrong, 1983). The geochemical signature of Dashwood has an elevated concentration of metals compared to Quadra Sand (Knight et al., 2015). Dashwood Drift was deposited during the penultimate glaciation early Wisconsinian, as deduced from the presence in overlying sediments of marine shells which date beyond the range of radiocarbon dating (Clague, 1980). |
| Description | The unit is composed of sandy silt and gravel, commonly with reddish oxidized hues and rich in fossil plants (Armstrong and Clague, 1977). Below these silts are clayey silt and sand with marine shells under which muddy diamicton assemblages and glaciomarine sediments are found (Armstrong and Clague, 1977; Fyles, 1963; Hicock and Armstrong, 1983). |
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