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NatureSpeak Articles

Rocks and Runs: The geology of Whistler Mountain

Shale slope pictured at right

Not surprisingly very few ski runs are named after rocks. For the same reason I suspect that very few swimming beaches are named after sharks. Among the 200+ named runs on Whistler/Blackcomb are tributes to deserving skiers and riders, past and present; to an ark-full of animals and birds, large and small; to teachers, loggers, truck-drivers; to culinary failures and even bathroom fixtures.

But rocks? Only one, the aptly-named Shale Slope. Unlike the more glamorous pretense of other named runs, the Shale Slope is in fact just that, shale. With summer hiking about to replace skiing this seems a good time to reflect on how this rock which began as mud on the bottom of an ancient ocean came to form one of the best bump-runs on Whistler Mountain.

The shale on Whistler is not, of course, confined to the "Shale Slope". It also forms the benchlands around Pikas and the upper reservoir, and much of the rock along the Harmony Loop trail. Nor is shale the only type of rock on Whistler. In fact volcanic rocks, andesite and dacite lava flows, form most of the mountain. Together the shale and the lava belong to an assemblage of rocks known as the Gambier Group which formed in a shallow marine basin during the Early Cretaceous Period about 100 million years ago. Silt, clay and fine sand, carried into the ocean by Cretaceous rivers, was deposited year by year, layer by layer, and compacted to form the thinly bedded shale. Simultaneous eruption of lava produced chains of volcanic islands and submarine flows.

Since the rocks of the Gambier Group were deposited they have been deformed, crumpled and uplifted by the slow but relentless pressure of convergence between the continent and the crustal plates of the Pacific Basin. The massive piles of lava that once formed the volcanic islands and submarine flows yielded by fracturing into large competent, mountain-sized, blocks while the weaker, thinly laminated shale was squashed, folded, and squeezed in between.

Look carefully at the edge of the black slabs of shale around Pikas. The subtle banding is caused by differences in the grain size or composition of the thin silt layers. The originally horizontal layering, now clearly folded and cut by numerous fractures, records the sedimentary origin of the rock and its subsequent deformation. If you are lucky you may find the carbonized imprint of a plant that washed into the ancient ocean basin where it was covered by silt and preserved as a Cretaceous fossil.

As future runs are cut and named and old runs renamed we are not likely to see a "lava leap" or "dacite dive", but at least the Shale Slope is a reminder that under the snow the mountains are indeed made of rocks with a long history deserving of recognition.

Written by: Jack Souther

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