Roche moutonnée glacial rock mounds - A remnant of the Ice Age, and nature’s jump ramps

Roche moutonnée glacial rock mounds with their gently dipping, scratched and polished upper surface and abrupt, jagged lower cliff face, quarried by Ice Age glaciers. Photo by Steve Carney

It’s that time of year again—the snow is almost gone, exposing the stunning glacial

landscapes around Whistler and the Sea to Sky corridor. One of my favourite viewing

spots is the Tantalus look-out just north of Squamish, where you can see a spectacular

selection of glacial features like jagged mountain peaks (horns), glacial ridges (arête),

mountain bowls and dramatic, steep-sided U-shaped valleys. If you look closely you can

also see lots of distinctive rock mounds, called roche moutonnée, with smooth, rounded,

uphill slopes and steep, rough and irregular downhill surfaces.

These hummocky features come in all sizes from a few metres to hundreds of metres in

length and are popular with skiers, snowboarders and bikers who enjoy the thrill of “big

air” off these natural ramps.

The somewhat wacky name roche moutonnée was first used by Horace Bénédict de

Saussure in the Swiss Alps, 240 years ago, when modern geology was in its infancy.

Their asymmetrical shape reminded de Saussure of the fashionable fleecy wigs people

were wearing at the time which (believe it or not) were smoothed over with sheep fat,

hence roche (French for rock), and moutonnée, derived from mouton (French for

mutton). So roche moutonnée means “fleecy rock.”

Despite their dodgy name, roche moutonnée outcrops are full of interesting features that

give us a tantalizing insight into how they were formed and even the direction of ice

movement.

The gently dipping uphill surface is created by “abrasion,” where rock fragments

embedded in the base of the glacier rub against the bedrock as it passes by, wearing it

away over time. Larger rocks scour and scratch the surface, leaving linear scratch

marks called “striations,” and fine grit particles polish and smooth the surface similar to

how sandpaper smooths wood. The steep, jagged lower cliff face is formed in areas

where there are weaknesses in the rock surface. Here glacial meltwater finds its way

into cracks and joints in the bedrock and freezes around chunks of rock that are forcibly

“plucked” out and carried away by the glacier as it relentlessly moves down the valley, a

process geologists call “quarrying” which produces steep, lower cliff face.

There are lots of fabulous and iconic roche moutonnée outcrops around Whistler and

they are pretty much everywhere. Some of my favourites are around Wedgemount Lake

where the granite rocks are freshly exposed by glacial recession, and on Cougar

Mountain around the 1,400- to 1,500-metre mark where they are covered in a thick

carpet of moss and look otherworldly. There is also a beautiful, classic-looking roche

moutonnée next to the Fitzsimmons Walk townhouses north of the village. Finally, at the

Malamute pull-in just south of Squamish, there are some superb roche moutonnée

which geologists have called “whalebacks” because the outcrop surfaces look like a pod

of whales swimming on the surface.

So, spring has sprung, and it's time to get out and explore the geology around Whistler

and our beautiful environment, carved out by Ice Age glaciers leaving behind a whole

smorgasbord of glacial features within a dramatic glacial landscape. A good place to

start is the distinctive glacial hummocks weirdly called roche moutonnée.

Written by Stephen Carney

#geology