Wild Northerner Magazine Summer 2017 - Page 63

By Back Roads Bill

For Wild Northerner

Humans sometimes exhibit erratic behaviour, but even rocks can do so.

Boulders of a particular rock type, which can weigh many tonnes, may be found sitting on a different type of bedrock; sometimes they are perched prominently in open areas. For the most part, they tend to be rounded. The surface is often fine grained, indicating grooves and scratches, showing remnants of the movement of the ice sheets and the constant scoring by neighbouring boulders. After the ice melted, the boulders remained distributed in a random way.

Most parts of northern Ontario have been glaciated - that is, ice sheets once enveloped the landscape. Glacial striae (you can see these scratch patterns on the bedrock), and glacially-oriented features, such as drumlins and eskers, indicate that the general direction of ice movement was towards the southwest. So as a result, most areas have erratic boulders.

Dr. Eric Mattson was contacted at Nipissing University. His speciality is snow and ice hydrology; glaciology and glacial geomorphology. He provided an “erratic” primer.

“The term ’erratic‘ is commonly used to refer to rocks that have been transported by glacier-ice to locations with differing lithographic character,” said Dr. Mattson. “They are one of a series of indicators which mark the path (direction) of past glacier movement. An examination of their mineralogical character leads to the identification of their parent bedrock sources, allowing for confirmation of the ice flow route.”

He said, “The size and shape of erratics vary tremendously. They can range in size from small pebbles to large boulders as big as a house, and their shape can vary from being very angular to round. These characteristics are a function of the original size and shape of the rock when it was entrained into the glacier system: whether it was transported on top, within or underneath the glacier, its mineralogical character, whether melt-water was involved and the distance which it travelled.”

He explained the most recent ice age as it applies to northern Ontario. “The Wisconsin Glacial Episode was the most recent major advance of the North American glacier complex, of which the Laurentide ice sheet covered northern Ontario. The maximum ice extent occurred approximately 25,000–21,000 years ago. This advance erased all the previous Illinoian glacial topography that extended over this area, hence all erratics found here belong to the Wisconsin Glaciation.”

Dr. Mattson identified the Okotoks Erratic found in the foothills of Alberta as one of the largest in Canada. It is known as “The Big Rock." The Okotoks Erratic is the largest known rock in the Foothills Erratics Train, a group of rocks that were carried by ice along the mountain front and let down as the glacier melted more than 10,000 years ago. These erratics lie in a narrow band extending from Jasper National Park to northern Montana. The Okotoks Erratic weighs an estimated 16,500 tonnes. It measures about 9 metres high, 41 metres long and 18 metres wide. The rock has broken into pieces, but is still a large landmark on the flat prairie. What about northern Ontario?