Glacial polish on summit of Snake Mountain. Monkton quartzite polished ever so smooth by glaciers.

A feature occasionally associated with glacial striations is glacial polish. When fine grained sediments are drawn over bedrock they can leave an extremely shiny polish along with striations. The polish is a surficial feature and erodes easily so it can be harder to find than striations are. There are some good examples up at the summit of Snake Mountain. I’ve seen them repeatedly on harder rocks (Monkton Quartzite seems to be a good one; MQ is the bedrock at the summit of Snake Mountain and along the shore of Shelburne Bay park where the other photos in the gallery below are from). I haven’t seen them on shale, dolostone or marble, but likely because these features erode rather easily and so don’t hold that thin layer of polish for very long.

Underside of the Lessor’s Quarry Thrust slickenside.

This feature can look strikingly similar to slickenlines – the sort of corrugated polish that forms at the contact point of a fault where one slab of bedrock moves along another, polishing both. With glacial polish – because one of the surfaces is the glacier, the polish is only on one rock surface. Look for these at thrust faults (like the overthrust at Rock Point; there are also some good examples at Lessor’s Quarry). The image to the right shows the striations on the under surface of the Lessor’s Quarry Thrust. It’s hard to see the polish, but there were parts where the rock was ground smooth.

 

More on the topic

Categories: Geology, Glaciers

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