The Nanaimo lowland on eastern Vancouver Island is completely dependent on groundwater for its potable water. The groundwater occurs within a complex mixture of unconsolidated sediments and fractured bedrock which is poorly understood. The Geological Survey of Canada is assessing this
resource by mapping the three dimensional geometry of the unconsolidated sediments and bedrock fractures, and by hydrogeological modelling. This is a short description of the surficial materials comprising the study area based on new surficial mapping, boreholes, seismic reflection surveys, and
existing water well records.
The overburden is generally over 100 metres thick in places and the basement contacts can lie below present sea level. Nevertheless, the thickness of the unconsolidated sediment is also variable, as indicated by numerous bedrock outcrops throughout the area. The surficial materials overlying the
Nanaimo lowland record a long interplay of glacial and nonglacial conditions coupled with fluctuating sea levels.
It is well known that this area was overridden by glaciers at least twice, with intervening non-glacial deposits found in places (Dawson, 1890; Clapp, 1914). Glaciation of east-central Vancouver Island involved two main sources of ice: local valley and piedmont glaciers, emanating from the
northwest-trending Island Ranges, and secondly, the Cordilleran Ice Sheet flowing off the mainland Coast Mountains. The configuration of the penultimate, Dashwood advance is not known, but it probably had a similar character to the final, Vashon advance. During the latter advance, a piedmont lobe of
the Cordilleran Ice Sheet flowed southeastward down the Strait of Georgia and impinged the eastern flank of Vancouver Island where it made contact with the local glaciers. Fossiliferous glaciomarine deposits associated with both glacial advances indicate high sea levels during both the advance and
retreat phases. The Quadra Sand, an extensive proglacial outwash unit deposited during the advance of the Vashon advance down the Georgia depression, is one of the most important aquifers in the Nanaimo lowland. During the maximum of the last glaciation, the Cordilleran Ice Sheet surmounted most of
the Island Ranges and flowed southwest across Vancouver Island. Deglaciation was marked by thinning of the ice and finally separation of the Cordilleran lobe from the Island valley glaciers. Once separation occurred, ice-contact glaciofluvial terraces and deltas were deposited along the retreating
ice margins. Eventually the sea flooded in from the southeast and glaciofluvial deltas were built into a high postglacial sea, reaching 150 m above sea level. In some valleys, such as the Englishman River valley, the local valley glaciers seem to have persisted and even readvanced after the retreat
of the Cordilleran lobe. With complete deglaciation, sea levels fell rapidly and the glaciofluvial deltas and terraces were left perched on the mountain flanks. Where there was adequate sediment supply, the main rivers built deltas 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. But, below the marine limit, the till is 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.