Toronto

How did a boulder over a billion years old wind up on Toronto's Bloor Street?

At first glance, it might look like it came from outer space, but experts say a giant rock unearthed during the construction of a downtown Toronto parkette originated right here on Earth, where it's believed to date back over a billion years. 

Photos of the 2,000-kg granite rock have been cropping up on social media

The 2,000-kilogram granite boulder turned up during the digging for a parkette in the Bloor-Annex business improvement area — and is estimated to be 1.1 to 1.4 billion years old.  (Joe Fiorino/CBC)

At first glance, it might look like it came from outer space, but experts say a giant rock dug up during the construction of a downtown Toronto parkette originated right here on Earth, where it's believed to date back over a billion years. 

The 2,000-kilogram granite boulder turned up during the digging for a parkette in the Bloor-Annex business improvement area — and is estimated to be 1.1 billion to 1.4 billion years old. 

"It gives us a link to the past," said Brian Burchell, chair of the BIA.

University of Toronto professor Joe Desloges says the rock very likely originates from the Canadian Shield, something that's not terribly unusual to come across in the Greater Toronto Area. Shield rocks are commonly found embedded in Toronto's many creeks and ravines, he says. 

But how this particular boulder ended up on Bloor Street West is something of a mystery.

Desloges says the rock likely travelled to the area from Georgian Bay around 14,000 years ago, when the glaciers that covered what is now Toronto began melting as the Ice Age ended — slowly exposing what is now Lake Ontario and Lake Erie as they retreated.

His belief: the boulder was likely frozen inside an iceberg, settling on the bottom of Lake Iroquois, Lake Ontario's much larger glacial predecessor, when the iceberg melted.

"The only way you're going to get something of that size deposited in those materials is if an iceberg were floating around in glacial Lake Iroquois, melted slowly, and these large pieces of shield rock would have deposited out from the bottom of the icebergs and dropped down into the soft lake bottom sediments," he said. 

So what do you do with a geological relic over a billion years old?

Plans for the parkette on Howland Avenue and Bloor Street happened to involve bringing granite in from Quebec anyway.

"But having something local and something related to the glacial legacy of the area, I think that's great," said Desloges.

"And [it] will give people a sense of what glaciers were capable of doing many years ago."