Geology of Sargeant Bay Provincial Park by John Newell
The present landscape of Sargeant Bay has been much modified by glaciation during the Ice Age that ended only 10,000 years ago. There are two topographically distinct regions in the park. Northeast of Colvin Creek there are extensive outcroppings of diorite, with only a thin, discontinuous veneer of overburden. In the area of more subdued topography to the southwest, the granitic rocks are covered by unconsolidated sediments of Pleistocene age (1.6 million – 10,000 years before present). The most extensive younger sediments are Capilano marine and glacial outwash deposits derived from reworking of earlier glacial tills. They have been eroded from the lower part of the Colvin Creek valley, exposing the underlying pre-Vashon sediments, the product of an earlier Pleistocene glaciation. Examples of these glacial sediments can be seen on the north side of Redrooffs Road, opposite the park gate. Here they consist of poorly sorted, gravelly glacial till containing a diversity of pebbles, overlying a bed of fluvial sand.
As everywhere, the scenery we enjoy at Sargeant Bay is closely tied to its geology. In the age of dinosaurs, some 150 million years ago, the west coast of North America was well to the east of its present position. The oldest rocks on the Sechelt Peninsula formed in an offshore volcanic island arc, much like Japan and the islands of the eastern Caribbean. Subsequently, the endless movements of the Earth’s tectonic plates carried these rocks downward, deep beneath the edge of continental North America.
Under intense heat and pressure they melted and recystallized to create the granitic rocks that now form the Coast Mountains. Remnants of the original island arc are widely preserved. Locally, the Caren Range provides many examples .
A stroll along the beach at Sargeant Bay illustrates the many variations in these rocks. There are dark pebbles and light pebbles, some coarsely crystalline, others fine grained.
However, the bedrock exposed within the park, from the prominent bluff at the east end of the berm, northwards to Triangle Lake, shows relatively little variation. It is classified as diorite, one of a number of igneous rock types loosely referred to as “granite”. It is comprised of medium-grained, interlocking crystals of the black mineral hornblende and greyish white plagioclase feldspar .
south of Triangle Lake
Small slivers of the original volcanic and sedimentary rocks can be found within the diorites on the ridge south of Triangle Lake. They are dense, fine-grained, dark grey rocks, called hornfels.
Some show relict sedimentary banding, reflecting their origin as mud and silt deposited on an ancient seabed.
The wetland, arguably the most important feature of the park, is itself a product of continuing geological processes. Capilano sediments, forming the bluff west of the bay, are easily eroded. Wave action carried the coarser material towards the head of the bay, where it accumulated to form a barrier bar protecting a tidal lagoon. The buildup of gravel eventually choked off the entrance and silt flushed down Colvin Creek was trapped behind the berm. Silting up of the lagoon allowed freshwater marsh vegetation to become established, creating an ecological system that is unique on the Sunshine Coast.