Trip to Cantley Quarry

As a graduate student part of my responsibilities besides lab work, data management and beer drinking are acting as a teaching assistant for courses during the academic year. This year I am TA’ing an introductory geology course. Part of the course involves field trips (we would be a pretty crappy geo department if we didn’t take students to the field) to various locations in the Ottawa area. Our most recent trip was to Cantley Quarry in Cantley, Quebec to look at evidence of glacial erosion caused by the Laurentide Ice Sheet during the Pleistocene (see my earlier post on the Pleistocene). The effects of the glaciers are pretty amazing and evident on large scale at Cantley. Here are a few photos taken by Roshenek Sonei, my co-TA.

Huge glacial erosion feature. This is a rat-tail in marble. The gneiss xenolith is to the right of the photo and looks rusty.

 

Another view of the massive rat-tail

Smaller, more well defined rat-tails within a larger rat-tail.

Rat-tail on a vertical section. Note the grooves in front and to the sides of the raised area. Glacial movement was from right to left.

The rock that the glaciers have eroded at Cantley is marble that contains gneiss xenoliths. The gneiss xenoliths are much harder than the surrounding marble and therefore did not erode as easily. This fact is what created the spectacular shapes at Cantley. When the glacier was present at Cantley there was sediment rich water flowing underneath it at high pressure. This water acted like a pressure washer might on your deck. It cleaned everything off the rock and eroded the marble. However, the marble behind the gneiss xenoliths was somewhat protected from the flowing slurry of destruction beneath the glacier and was not eroded. This is why there are so many odd ridge shapes at Cantley. These features are known as rat-tails. A good analogy for the formation of a rat-tail is to look at the way water flows around a bridge pillar. When the water hits the pillar it is pushed back away from the pillar and then wraps around it leaving the area directly behind the pillar protected. The xenoliths acted the same way protecting the marble directly behind them but causing erosion at the edges of the protected area and directly in front of the xenolith.  The other hypothesis for the formation of rat-tails involved direct abrasion by the glacial ice itself, however this theory does not explain the odd direction of some the rat-tails at Cantley as the glacial ice would not have been able to change direction. This suggests the rat-tails were more likely formed by the slurry of destruction that occasionally changed direction due to changes in the pressure regime under the glacier which would be the result of the glacier’s movement or changing size.

One interested piece of information that can be gleaned from looking at rat-tails is the direction the glacier was moving in. This is evident since the xenolith protects the marble behind it. Therefore the glacier must have encountered the xenolith first meaning we can figure out which direction the glacier was moving. At Cantley the direction of glacial movement was roughly from north to south.

Another beautiful glacial feature found at Cantley are striations which are caused by rocks stuck to the bottom of the glacier that are dragged over the rock creating long scratches and grooves. These striations can be several metres in length and create a surface akin to ice after a hockey game. At Cantley the white marble makes it look exactly like that.

Glacial striations

Thanks for reading and if you have any questions or comments about glacial features or glaciers feel free to post them.

Matt