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Site Profiles: Okotoks Erratic

Long distance view of the Okotoks Erratic

It looks as though it fell from the sky and cracked on impact.There's nothing like it anywhere in the vicinity; unless youcount those large protuberances spearing the horizon far off tothe west.

The large rocks spearing the horizon - the Rocky Mountains,as they are known - provide the clue. The three-storey-highboulder sitting all by its lonesome in a field near Okotoks ismade of quartzite. So, too, in places, are the Rocky Mountains,notable near Jasper where most of Mount Edith Cavell and PyramidMountain are composed of quartzite. But Jasper National Park is400 kilometres (248 miles) northwest of Okotoks. How could amammoth piece of quartzite travel from point A (Jasper) to point B(the grasslands south of Calgary)?

Simple. It hitched a ride.

Okotoks ErraticQuartzite is the hardest rock in the Rockies, harder even thansteel. It was composed in the Cambrian period, long, long beforethe Rocky Mountains appeared, 570 to 525 million years ago whenlife in the form of primitive sea creatures exploded in the warmclear waters that marked Alberta's west coast. This ancientseabed was then buried deep under sediment, compacted overmillions of years until, through tremendous heat and pressure, itturned to quartzite. Four hundred million years later, when theRocky Mountains began their upward thrust, the quartzite sawsunshine for the first time.

Though very hard, quartzite is also brittle. At some pointduring our glacial age, geologists believe, a rockslide occurredon a mountain peak in the Jasper area, sending huge pieces ofquartzite tumbling onto a glacier that happened to be passingthrough the Athabasca River Valley at the time. As it creptforward, the glacier carried the enormous rocks eastward towardthe plains until it banged into the western extremity of thecontinental ice sheet. The glacier then veered southeast, toparallel the eastern face of the Rocky Mountains.

Okotoks ErraticWhen the ice started to melt and the glacier began its retreat,the rocks, known as "erratics", were left behind in aribbon extending from Jasper along the western prairie marginsouth to northern Montana, roughly the contact zone between thecontinental and mountain ice sheets. Of this group, knowncollectively as the Foothills Erratics Train, the 16-tonne (17.6ton) piece at Okotoks, dropped at its final resting place about12,000 years ago, is by far the largest; indeed, it is consideredNorth America's largest glacial erratic.

The town, by the way, got its name from the rock. Okotoksmeans, not surprisingly, "big rock" in the Siksikalanguage.

To read more about the legend of the Big Rock, click here.

GETTING THERE: The Big Rock at Okotoks is visible on the northside of Highway 7, eight kilometers (five miles) west of thejunction of Highway 7 and Highway 2A. The town of Okotoks isapproximately 30 kilometres (19 miles) south of Calgary on 2A.

Reprinted from Barbara Huck and Doug Whiteway's InSearch of Ancient Alberta with kind permission from HeartlandAssociates, Inc.

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