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NatureSpeak Articles

Glacial abrasion and quarrying

The work of ice sheets and smaller glaciers in the development of the topography about Whistler is more than just scarcely recognizable. On the grandiose scale they have eroded valleys into that classic U-shaped profile; they have quarried out the alpine basins we ski in, and have produced the series of ridges that define the skyline. Whistler Bowl, West Bowl, Horstman Glacier Bowl, Harmony and Symphony Basins have been scooped into their present shape by glaciers which methodically, over thousands of years, have plucked at the bedrock and then ground the pulled-out fragments into smaller pieces with movement of ice from headwall to outer basin edge. The bowls, or "cirques" in geological parlance, were fashioned in the initial stages of the build-up of the Cordilleran Ice Sheet which eventually covered all but the highest peaks of the area about 15,000 years ago.

The relentless movement of the ice sheet towards the Straight of Georgia has abraded all rock surfaces, producing a series of features that are easily seen on any exposed smooth slab of bedrock. Scratches on the rock, or striations, are produced by hard rock particles in the ice that are dragged across the underlying bedrock, and thus show which way the ice sheet was moving, as seen so well along the newly exposed surfaces in the highway work at Cheakamus Canyon. Moreover, some of the rock is actually polished by the finer particles in the sole of the glacier ice.

In areas of "knobby" terrain, such as the summit plateau of Cougar Mountain, the hummocks will have a smoothly abraded "stoss" surface and a ragged leeward side, plucked out by ice, giving rise to the name "roche moutonée" (stone sheep). On the nature trail in Lost Lake Park there is a fine example of such a feature, which also has a trailing ridge of loose rock debris tapering "downstream" (or to the south) giving rise to the term "crag and tail" drumlin.

The upward extent of the various features of abrasion and quarrying and rubble left in the wake provides a good hint at the elevation of the ice sheet at its maximum phase of accumulation about 15,000 years ago. Plucked features and striations can be found on the summit of Whistler Mountain (2,160 m) but not above the Horstman Hut (2,252 m) on Blackcomb Mountain (2,437 m). Angular rock rubble above the hut and extending to the summit was produced by severe frost activity. The surface of the ice in this area was likely just below hut level.

One of the best areas to observe the effects of glacial abrasion is the motorcyclists trail on the south-east side of Cougar Mountain. The riders have chosen a path which runs over relatively smooth slabs of sloping granitic bedrock. The trail begins at the "Welcome to Whistler" sign a hundred or so metres north of the heliport. It crosses the Thrill Me-Kill Me mountain bike trail and climbs the mountain on some incredible grades. Underfoot the rock is adorned with striations; here and there are crescent-shaped gouges; that is, a set of steps chiseled into the rock, and elsewhere there are minute arc-shaped fractures, or chatter-marks, caused by the irregular bumpy motion of the ice sheet.

At about 1,300 metres elevation the bikers’ trail meets an abrupt end at a cliff band, but a narrow gap cutting through it, marked with ribbons, leads to a long narrow lake basin honed out of the bedrock by the ice sheet. Giant "stoss and lee" rock hummocks mark the entrance to the basin. Walking around the west side of the lake and then ascending slopes obliquely to the east will lead to the highest hummocks of Cougar Mountain, which is actually a rocky plateau (1,450-1,500 m elevation) with many small rock lake basins plucked and honed out by the overriding ice sheet.

Nearby, a spectacular wall of glacial striations and rock polish can be seen from the summit. It appears that the ice stream of the Soo Valley overrode Cougar in a sweeping bend of movement toward the Green-Alta Lake basin, and thence seaward.

It’s worth the hike to view not only the handy work of a 15,000-year-old ice sheet but also marvel at the rock surfaces that skilled motor cyclists are ascending! Step aside if you hear them coming!. They will be flying.

Upcoming Events:

Monthly Bird Walk — The next bird walk will take place Saturday, Oct. 2. Join Whistler experts in the monthly update of our feathered locals and migrants.

Written by: Karl Ricker


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