Rick Hudson - travel writer and author

This article was written for the Canadian Rockhound On-line Magazine
Spring 2001 edition.

A brief geology of British Columbia
Rick Hudson

The Province can be divided into four major physiographic segments. Reading from left to right on the map, they are the Western, Central, Eastern, and Alberta Plains divisions.

The Western System is made up of two ranges: the outer mountains of the Queen Charlotte Islands and Vancouver Island, and the inner Coastal Ranges that back onto the City of Vancouver, and run up the coast to the Alaskan Panhandle. The two ranges are separat-ed by the Coastal Trough (Georgia Strait up to Hecate Strait). This is still a geologically active area. In the past 10 million years, young volcanoes have formed along the margin at places like Gari-baldi Mountain and Black Tusk.

The Central System is a blend of plateaus and mountain areas which formed from about 200 million years onwards. Its shape today is the result of recurring glacial ice, as well as volcanic activity. By and large, the mountainous areas are underlain by old rocks, including igneous, metamorphic and Pre-Cambrian sedimentary. The plateaus, on the other hand, are generally younger. Historically, this has been a great mineral and mining region, such as the lead-zinc deposits at Trail, the copper ores of Logan Lake, etc.

The Eastern System is quite different. It is made up of the Rocky and Mackenzie Mountains, and is bounded on the western side by the Rocky Mountain Trench, which has the towns of Kimberley and Golden in it. Gran-ites are uncommon. Instead, the records show massive piling up of slabs of rock that were pushed from the west, mostly during a time of between 100 and 40 million years ago.

Calcite veining in limestone
Central System mineralization.

This created multi-layers of sedimentary rocks, such as Mt Robson near Jasper, and all the peaks along the Divide in Banff-Jasper-Yoho-Kootenay National Parks. In some places, old rocks have been pushed up over younger ones. Generally, there is a tendency for the south west slopes to be gentle, the north east ones to be steep or even cliff-like, resulting in grand scenery. This is fossil country, such as the World Heritage site at the Burgess shale beds near Field.
The Alberta Plains is a sedimentary basin that intrudes into the north eastern corner of the province, on the east side of the Rocky Mountains. Drained by the Peace River, sediments here are thousands of metres thick, and are known best for their oil and gas production. Fossils abound.
Igneous rocks:
Igneous rocks are those that have come from molten matter deep inside the earth. They may be intrusive, which means they never reached the surface, but squeezed into cracks on the way up (as dykes or sills), and were later exposed due to weathering or uplifting. Granite is a typical intrusive rock, being coarse grained. It cooled slowly deep down, allowing feldspar, quartz and usually mica and hornblende to crystallize in the rock matrix.
The mountains in the Bugaboo Range (to the west of Windermere) are granites, as is the Sawamish Chief, that huge cliff that overhangs the highway as you near the town of Squamish, at the top of Howe Sound. In the interior, the porphyry copper deposits of the Highland Valley Mine (until recently the largest operational open pit in North America) near Logan lake are the result of a Triassic era batholith (intrusive granitic mass).
Igneous rocks can be extrusive too, which means they reached the surface as lava or magma flows and cooled quickly, to form fine grained material. The lava flows below Black Tusk and Garibaldi Pk, above Highway #99 are this type, where you would expect to find obsidian, basalt, and rhyolite. Basalt and rhyolite have much the same chemistry as granite but, by cooling more quickly, do not allow crystals to form. When rhyolite is blown by hot gases into a sponge texture, it is called pumice, which is a volcanic froth. There are pumice deposits NW of Pemberton.
There are a wide variety of extrusive rocks in British Columbia. The most important, from a rockhounding point of view, are the basalts, which are commonly vesicular (full of voids). These gaps may later fill with agate, calcite or zeolite minerals, to become amygdaloidal lavas. The most prolific flows are those that formed during the Tertiary Age (65-20 million years ago). Many rockhounds have hunted the agate beds of the Kamloops-Vernon-Princeton region. Areas to the N have similar geology, but have not been explored much. Many regions are unfortunately covered by overburden (often glacial till).
Sedimentary rocks:
Sedimentary rocks are varied, and are produced by fluid (usually water, but sometimes wind) depositing them in lay-ers. They therefore form in strata or beds, and are initially flat. They develop from pre-existing rocks through erosion, and can be fine grained (like the sandstone beaches found along the shores of the Gulf and Georgia Strait islands), or coarse grained conglomerates (which look like piles of river pebbles glued together with concrete). The Rocky Mountains of the BC-Alberta border are fine ex-amples of sedimentary processes, with their obvious 'layer cake' look, often tilted due to recent thrusting.
Some sedimentary rocks are formed by chemical precipitation (gypsum), while others evolve from the compaction of marine ani-mal shells (limestone). A lot of chalk and limestone cliffs (including the famous White Cliffs of Dover in England) are actually trillions of marine organisms piled into layers, often hundreds of metres thick.
Metamorphic rocks:
The third type is metamorphic rock, and refers to any rock that has been changed after its formation. Thus, both igneous and sedimentary rocks can be metamorphosed into something new. The change can be slight (slate is simply shale that has been heated), or it can be so dramatic you can't tell what the original material was (sometimes know as fubarite = 'fudged up beyond all recognition'). Pres-sure, chemical penetration, and heat are the great metamorphic forces. Marble is metamor-phosed limestone, and quartzites were usually sandstones once. Schists and gneisses are often heavily changed shales and granites, after other material has invaded them.