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The major geomorphic features of the Nanaimo Lowland are the product of structural, erosional, and
depositional processes. Folding and faulting of the bedrock, erosion and repeated glaciation, as well as
isostatic and eustatic changes of sea level, have all contributed to the physiographic features of this
region.
Differential erosion of bedrock throughout this physiographic zone has produced a distinctive pattern of
cuesta-like landforms where areas underlain by competent sandstone, conglomerate, and volcanic or
intrusive igneous rocks form ridges, and soft shale, mudstone, or areas with intense bedrock fracturing
form bowls or valleys. In areas predominantly underlain by metamorphosed granitoid rocks such as to the
east, west, and south of Shawnigan Lake, the terrain tends to be more rugged with the development of
steep, conical hills and bedrock fracture-controlled valley lineaments. The bedrock surface between the
north end of Shawnigan Lake and Cowichan Bay has been extensively modified by glaciation, which
deposited a thick mantle of heterolithic debris over most of the area during the glaciers’ advancing and
retreating phases.
2.3
Bedrock Geology
Rocks from two discrete geological provinces, known as the Wrangellia and Overlap Terranes, underlie
most of the South Cowichan area (BCGS, 2008). The boundary between the Wrangellia and Overlaps
Terranes is represented by a northwest-trending erosional unconformity that runs south of the Cowichan
Bay area and meets the marine shoreline at Cherry Point. Rocks from a third discrete geological
province, known as the Crescent Terrane, underlie the southernmost tip of the Study Area at the
Goldstream River estuary. This terrane is separated from the Wrangellia Terrane by the regional-scale
Survey Mountain Fault.
The Overlap Terrane consists of sedimentary rocks of the Upper Cretaceous Nanaimo Group (see Figure
4).
The Nanaimo Group in the South Cowichan area is represented by the Comox and Haslam
Formations, a conformable sequence of marine and non-marine sedimentary rocks that grades upwards
from carbonate-rich deltaic sandstone and conglomerate, through rhythmic marine beds of siltstone,
sandstone, and coal-bearing shale, into pure shale and mudstone. Nanaimo Group rocks are only rarely
exposed within the South Cowichan region due to their deep burial by glacial sediments, but are often
encountered at depth by drilled wells throughout the Cowichan Bay and Cherry point areas.
The Wrangellia Terrane is represented by a range of igneous, volcanic, and sedimentary rocks of various
ages within the Horne Lake – Cowichan uplift (see Figure 4), one of a number of northwest-trending
geanticlines within the Wrangellia Terrane that make up the structural backbone of southern Vancouver
Island. Stratigraphic components within this terrane are as follows:
The central core of the Horne Lake – Cowichan uplift in the South Cowichan area is formed by
Palaeozoic-aged, granitoid rocks of the Westcoast Crystalline Complex, and includes the Wark and
Colquitz Gneiss Formations (see Figure 4). Westcoast Crystalline Complex rocks are commonly
exposed as conical hills throughout the southern half of the South Cowichan area, and along the
shoreline of Saanich Inlet from Bamberton to Goldstream;
