This is my first official post, besides the welcome post, at GeoSphere – EGU edition. It seems fitting to begin with a post that is part of a continuing series from my old home and is bridging the way to my new one. The photo of the week, while still only six weeks old, is and will stay a regular fixture on my blog. The photo for this week is of some fantastic glacial striations in glacially polished marble located in Cantley, Quebec, which is about a 40 minute drive from Ottawa and is one of the most popular field trip sites for students from uOttawa.
Glacial striations are an interesting feature. They are pretty much ubiquitous across Canada, but rarely are they so defined as they are in Cantley. Striations form from the abrading action of rocks and sediment particles embedded in the base of a glacier. As the glacier slides across the bedrock it scrapes these sediments along the rock below making these striations. In order to form a striation the glacier must be sliding, which only occurs when there is a little water at the bed-glacier interface to lubricate things. Furthermore, if you look at striations under high magnification you can see that the sliding is discontinuous. In fact, it is a series of perfectly aligned slips of only a millimetre or two that are connected to form a perfectly straight groove up to several metres long. Another great feature about striations is that they can tell us the orientation of glacial movement. Indeed, the parallel lines in the rock below point the way to glacier was flowing. However, it is very difficult to tell the glacial flow direction from a striation since the glacier could have been flowing in either direction to make a straight line. In Ottawa we know from numerous other glacial features that the Laurentide Ice Sheet was flowing roughly from north to south. During the Pleistocene epoch, about 10,000 years ago, glaciers were covering pretty much all of Canada as well as most of Europe. The ice sheet at Cantley was ~2km thick.
Glacial striations at Cantley Quarry, Cantley, Quebec. Click for a larger image. (Photo: Matt Herod)
Thanks for reading!
Matt