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Surficial geology and Pleistocene stratigraphy from Deep Bay to Nanoose Harbour, Vancouver Island, British Columbia

A detailed study of the surficial geology of the east coast of Vancouver Island from Deep Bay to Nanoose Harbour was undertaken as part of the Nanaimo Lowland Ground Water Assessment Project. This paper describes the results of the field mapping program and resulting surficial geology map.
The map incorporates Pleistocene stratigraphy derived from new seismic reflection surveys and boreholes carried out by the GSC, and lithological information extracted from BC Ministry of Environment water well database.
Large parts of the coastal lowland are mantled by unconsolidated material over 100 metres thick, but the thickness is quite variable and bedrock outcrops are common. The surficial materials are dominated by deposits from two previous glaciations that are separated by non-glacial fluvial and marine
sediments. During the last glaciation, the coastal plain was subjected to glacial advances from two distinct sources: local mountain glaciers expanding out of the Vancouver Island Ranges, and secondly, the Cordilleran Ice Sheet flowing from the Coast Mountains to the northeast. Quadra Sand, an
extensive proglacial outwash, was deposited over the coastal plain with the approach of the Cordilleran Ice Sheet. At its maximum the ice sheet flowed over the Island Ranges, over-topping the local glaciers.
Deglaciation was marked by thinning and eventual separation of the two ice masses and marine inundating of the coastal plain to at least 150 m above present. At this time extensive gravels were deposited along the ice margins in the form of ice-contact deltas and terraces. With final deglaciation
and rapidly falling base levels most sedimentation was concentrated at the mouths of the main rivers where deltas were built into progressively lower sea levels. In other places, transverse streams crossing the lowland either terraced pre-existing glacial deposits, or cut narrow canyons in the
bedrock. A cobbly surface till covers most the flat-lying interfluves except below the marine limit where the till was partially reworked by marine action or covered by marine sands and silts in poorly drained areas. Modern alluvium is confined to the lower reaches of the larger rivers.

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