McGregor Bay: A magical mystery geological tour with a french twist

Tuesday, September 20, 2016

Written by Peter Russell with introduction by Linda Lapointe

On the day after a filled to capacity Dessert with a Dash event on the geology of McGregor Bay, Peter Russell, Curator Emeritus, at the Peter Russel.University of Waterloo’s Earth Sciences Museum, generously agreed to lead a guided tour around the Bay. Despite the storm warnings on Tuesday, July 15th, we decided to go ahead with our tour and the heavy downpour rain held off until around noon – just when we had finished.

Photo courtesy of Betsy Frank.

We started at the Church docks and went through Rocky Bottoms into the east end of Iroquois Bay (North Bay). There were around 25 of us in several boats which gathered at different spots to hear Peter’s lively and enlightening descriptions. As anyone who attended the Dessert with a Dash presentation knows, Peter’s knowledge of, and, enthusiasm and love for geology as well as his down to earth manner helped us all to better understand McGregor Bay’s features. At the same time, some of us realized how little we know of this very complex topic and of our surrounding environment. However, at the end of the tour, many of us would never look at McGregor Bay in the same way.

Geological description of McGregor Bay

The rocky islands and mainland in McGregor Bay is the result of various forces – different layers of rock formed at different periods in the Earth’s history, folding of rocks as a result of the movement of the underlying tectonic plates, dikes formed by the movement of hot magma or lava from below the earth’s surface into cracks in existing rock structures, sedimentary rock left behind from an ancient sea (younger sedimentary rocks are found on Manitoulin Island), and, the advancing and retreating of glaciers during different periods.

Most of the rocks in McGregor Bay are from the Middle to Late Precambrian period 2.2 to 2.4 billion years ago named The Huronian (after Lake Huron) by Canada's first geologist William Logan who mapped this area by canoe. These Huronian sedimentary rocks were formed by the weathering of older rocks in northern Ontario and spreading debris south in a thick wedge shaped deposit thousands of metres thick towards southern Ontario. The deposits were small particles of sand from minerals or rocks (forming sandstone), mud (forming argillite or mudstone), pebble and

William Logan.

boulder layers (forming conglomerate), and, calcium from bacteria and simple plants creating lime mud (forming limestone). The rocks were laid down in cycles each starting with a glaciation which deposited conglomerates and ending with River deltas spreading sand into the sea forming sandstone. Parts of three of these cycles are present in the McGregor Bay Area - they are shown from bottom to top on the map below: the Hough Lake, Quirke Lake and Cobalt groups as described briefly below.

  • Hough Lake Group: Mississagi sandstone
  • Quirk Lake Group: Bruce Conglomerate, Espanola limestone, Serpent sandstone, Gowganda conglomerates and mudstone.
  • Cobalt Group: Conglomerates, mudstones and Lorrain sandstones and conglomerate made of white quartz and red jasper pebbles.

Cartoonish map of McGregor Bay area.

Geological Map of McGregor Bay Area.

This square represents the Huronian rocks before pressure from the north and south compresses it or shortens it, folding the crust.Mountain building movement provided heat and pressure changing sandstone to quartzite, cementing the grains together and folding the rocks forming folds with their crests in east to west direction in McGregor Bay.

Earthquakes like those shaking California from time to time form faults or fractures in the crust, some hundreds of kilometres long. The fractures allow volcanic rock (molten magma) easy
access, such as the Nipissing Diabase found throughout the area (pink on the map). The molten rock filled fractures in rocks across Canada about 2,217 to 2,210 million years ago.

Cracks (faults) in the Earth's crust caused by earthquakes are used as pathways for molten magma.

Erosion weathered away thousands of metres of rocks above what you see today. The latest erosion was caused by ice scouring the landscape during the ice age. The last ice melted away around 12,000 years ago. The ice with boulders and pebbles underneath ground and formed the shape of your islands (some look like whale backs). Scratch marks and large far traveled rocks in the landscape are a signature of the glacier’s passing. Some rocks weathered further after end of the last ice age. Other interesting things are found in the ancient rocks including ore deposits, some economic, some failed adventures.

Below are some of the rock formations we stopped at on our tour:

Rock formation with some water and flora.

Picture courtesy of Peter Russell.

The photo above illustrates the weathering around quartz veins in the Nipissing diabase. When the veins of quartz filled cracks in the diabase they heated a few centimetres of rock near the crack changing some of the minerals to a soft mineral called chlorite mica. This mica weathers away faster than the Quartz and diabase.

Collage of rock formations.

Pictures courtesy of Peter Russell.

Boudinage! In the collage of pictures above, the picture on the left shows the stretching of a quartz vein which caused it to break into pieces held together by a thin quartz vein like a string of sausages! “Boudin” is the French word for sausage!

In the picture in the middle, the squiggly quartz vein is about two metres from the quartz vein described above. Both were formed at the same time in an argillite (mudstone) and are parallel to the layering.

The picture on the top right is a conglomerate in the Gowganda formation described above.

The photo on the bottom right shows drag folding in the Serpent quartzite. Drag folds are one of the simpler types of folding. They represent the bending of rock before it breaks forming a fault.

Join us for another geological tour this summer. Peter will be doing another Dessert with a Dash plus a “field” trip (in boats) the following day (Tuesday, July 11th). Same station, different spots.

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