GLACIAL LINK TO BIG ROCK FOUND IN MISSION
The Okotoks ‘ Big Rock’ is not the only glacial erratic in this area. A condo construction site recently uncovered one in Mission.
It was supposed to be just another day on the job site, excavating some earth where an old house once stood.
When workers dug in, though, it was anything but a routine experience.
“It was really tough rock, about eight feet below grade,” says Josh Poirier, site and safety superintendent for the Tela condo project in Calgary’s Mission district. “The excavators brought in a hoe with a breaker on it. They spent about two hours trying to break the rock apart, but they only got about nine pizza box- sized rocks out.”
While Poirier and his team scratched their heads, they soon found themselves joined by many others.
“The president of the excavating company came out, they all said they hadn’t seen anything like this before,” he says.
They discovered that a massive rock — about four metres by four metres and weighing about 85 tonnes — had been under the house on 22nd Avenue S. W. for all of its 80 years.
Actually, the rock had been there much longer. About 12,000 years.
“The University of Calgary sent out some geologists who told us it was glacial erratic,” he says of the pink rock too heavy even for the biggest crane to lift. “None of the crane companies were even willing to try.”
While the discovery of glacial erratic in the heart of Calgary is a rare occurrence, it is certainly not without precedent. It is defined as glacier- transported rock that differs from the local bedrock. A glacial erratic can be as small as a pebble to bigger than a house, and may have travelled up to 1,000 kilometres from its original location. Their existence today helps tell the story of the retreating glaciers at the end of the last Ice Age.
Because it was deposited in the Elbow River floodplain, this particular glacial erratic has been left unnoticed thanks to being buried by thousands of years of sedimentation. Stellar examples of above- ground glacial erratics, though, abound in this part of the world.
Measuring nine metres high, 41 metres long and 18 metres wide, the 16,500- tonne Big Rock near Okotoks is not only the largest of the group of rocks that were carried by ice down the front of the Rockies and let down when the ice melted, it is also the largest in the world. It’s a member of the Foothills Erratics Train, a narrow band of glacial erratics that stretches from about 600 kilometres from Jasper to Montana.
Other notable examples can be found in and around Calgary, from the memorial plaque to John Laurie, a politician and aboriginal rights supporter, that sits atop a glacial erratic at Nose Hill, to Split Rock in Confluence Park just north of Beddington. The University of Calgary’s famed Rock, which has been painted over countless times, was glacial erratic that was found during the 1968 excavation for the school’s social sciences building.
Because of the long history of such a natural phenomenon, it’s not surprising that the blasting of the rock at the Tela condo project prompted one longtime Mission resident to write to the local community newsletter. It was penned after an article in the newsletter’s March issue described the discovery and subsequent breaking up of the rock.
Entitled Mission Hits Rock Bottom With Destruction of Erratic, the author asked why the rock hadn’t been moved to a nearby park, bemoaning the destruction of “a priceless treasure of our archaeological heritage more ancient than the Pyramids.”
Not all area residents agree with such sentiments.
“It’s just rock,” says Robert Jobst, a longtime community activist who has called the neighbourhood home for more than 30 years. “The story of how it got there is interesting, but there is nothing intrinsically interesting about the rock itself.”
Jerry Osborn, a professor of geology in the geoscience department of the University of Calgary, shares that view.
“It’s definitely not common to find them in Calgary excavations,” says Osborn, who notes another famed glacial erratic can be found in Airdrie, which has been designated a provincial historic site. “But I think it’s fairly unremarkable geologically.”
What Osborn does find remarkable is the glacial erratic found in South Saskatchewan. Like the one discovered in Mission in January, it’s also below ground.
“It’s 10 kilometres long,” says Osborn.
As for what’s left of the glacial erratic found at his condo site, Poirier says the bigger pieces are being stored at a facility in the Cochrane area. One piece of the dense rock known also as quartzite, though, will return, albeit in a new form, to the location where it’s been for the past few thousand years.
“We’re going to make a table out of it,” he says of the rock that took nearly two weeks to excavate. “We’ll put it in the lobby.”