“A Rolling Stone Gathers no Moss” but the stories they can tell…

Posted: April 3, 2012 - 09:51 , by admin
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Natural History, Canada | Comments (0) | Comment

Submitted by Vincent Vertolli, Assistant Curator Geology

In September of 1959 Dr. Walter Tovell, Curator of Geology was contacted by the operators of a limestone quarry to find out whether the ROM would be interested in a very unusual boulder they found. Normally when a geologist hears of a boulder, even an unusual one, the first thought to come to mind is “so what” because there are countless numbers of stones and boulders scattered throughout the fields of southern Ontario-just ask any farmer. In fact you would be hard pressed not to run into a boulder, some as large as a car anytime you try to dig a hole. Thankfully Dr. Tovell agreed to see this “special” boulder which came with a fascinating story of how it was found and what it had to say about Ontario’s geological past.

The story began on a typical working day at the quarry with the blasting out of a sedimentary rock called limestone. This material was then hauled to a primary crusher and then a pulverizer to eventually produce a powder which is used to make lime. This is not a small operation: the quarrying and crushing plant has the capacity of processing 350-400 tons of rock per hour. Needless to say these crushers are enormous machines powerful enough to easily smash apart solid rock but on this day the crusher abruptly stopped working. Mechanics and electricians were called in to find the problem and get the machine back into operation but try as they might they couldn’t find a reason. There was nothing left to do but to dismantle the entire crusher and what they found jammed between the huge steel plates was a boulder made of solid copper and weighing about 14 kg.

Once removed from the jaws of the crusher the operators couldn’t help wondering where it came from, there isn’t any copper in limestone which is composed of the mineral calcite (calcium carbonate) so they concluded that it must have come from the 6 metres or so of sand, gravel and boulders that overly the limestone. But no one had ever seen a boulder like this, not only in the quarry, but anywhere near it and it wasn’t until Dr. Tovell saw it that its history and story could be told.

Many of the boulders found in southern Ontario can be traced to having come from the Canadian Precambrian Shield and their presence is just one line of evidence in support of the “Ice Ages” when continent-sized glaciers advanced and retreated across most of Canada and parts of northern USA. What’s often difficult to determine is the direction of ice flow, and the vast majority of boulders can only be said to have come from somewhere to the north. However a boulder of native (pure) copper is extremely rare. It is only known to occur in the Lake Superior region especially the Keweenaw Peninsula of northern Michigan and Mamainse Point in Ontario. To find a native copper boulder in Beachville, Ontario not only provides dramatic evidence of glaciation but it also tells us the direction of ice-flow.

With smooth edges, the copper bolder has shallow marks where it was crushed at the quarry.

The 14 kg copper nugget removed from the crusher. The indentations left by the jaws of the crusher can be clearly seen.

 

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