If Rocks could Talk

Sep 16, 2010

Photo: courtesy of Dave Deacon

For most of us the rocks of Bon Echo Park represent an exotic and awe inspiring landscape but for multi-award winning geologist Dr. David Pearson, founder of Science North and Professor of Earth Sciences at Laurentian University in Sudbury, they tell a story of the formation of the earth that goes back billions of years.

Close to 50 guests gathered at the Barrie Hall in Cloyne on September 11 to hear that story explained by Pearson, who traveled five hours from his home in Sudbury specifically to give the talk and to visit Bon Echo park courtesy of the Friends of Bon Echo.

To the professional geologist, the sinuous patterns and layers apparent in the Bon Echo rock faces demonstrate the fact that the rocks are roughly one billion years old and, in fact, are what remain of the Grenville Mountains, a mountain range that once stood as tall if not taller than the Himalayas and which formed when the super continent Laurentia was slammed into by South America. These continental collisions took place as the result of the ocean floors sliding underneath the edges of certain continents. Pearson stressed the fact that the Grenvilles were the “longest mountain bolt that the planet has ever seen”. Their roots are the rocks that we now know and call the Canadian Shield.

Over time the tall peaks of the Grenvilles eroded away and the hot slushy material found 100km below the earth’s surface also pushed up the roots of the Grenville mountains exposing the rock that now surrounds us today. “We are looking at rocks that were subjected to temperatures of 1200 degrees centigrade and to pressures that you would experience at 20km beneath the surface. We have to realize that these rocks were at one time plastic and were pressure cooked.”

Another part of the story, which accounts for the formation of the east/west bays found in Mazinaw Lake as well as myriad other lakes on the shield, concerns the breaking up of the super continent Rodinia that existed one billion to 800 million years ago.

At that time convection currents moving underneath the continent caused its break up and the east/west fractures and faults which resulted account for so many of the east/west bays that we know today.

The third part of the story, which accounts for the length of Mazinaw Lake, resulted from the break up of the last super continent Pangaea, which split apart 180 million years ago into the continents we now know as North America and Africa. The pulling apart of Pangaea caused north/south stress patterns resulting in the length of Mazinaw Lake following a north/south orientation.

The final part of the story is the part that caused the extreme depth of Mazinaw Lake, 475 feet in some spots. This event occurred 25,000-125,000 years ago when most of North America was covered by a sheet of ice two kilometres thick. As the ice sheet melted and retreated from the cliffs of Bon Echo rock, huge icebergs the size of city blocks fell off the cliff, were embedded in the sediment below and created incredibly deep pot holes, which account for the lake’s extreme depth.

Pearson included in his talk various maps, diagrams and photos which helped listeners visualize these incredible stories.

What is important is that after hearing his explanation, one can experience Bon Echo Park in a different way and with new understanding.

In Dr. Pearson's words “What people see in a landscape depends on their personal area of expertise.” By generously sharing his expertise, he has no doubt enriched how many of us will now view the magical landscape that is Bon Echo.