Glacial Striations

As mentioned in the blog post about how glaciers move, all sorts of material gets entrained in glacial ice. The entrained materials range from small stuff (like the silts that give glaciers that characteristic blue color) to big chunks of rock (like those big oddball rocks scattered throughout our woodlands). Some stuff is carried on top of the glacier itself (like Big Rock up in Alberta, which tumbled down from a rock slide, landed on top of the glacier, and was ultimately moved by the flow of the glacier). Other materials are carried along on the bottom of the glacier. All of the grit riding along the bottom works to grind down big jagged mountains to gentle rolling hills, and if we get up close to the bedrock we can see lines scratched on its surface.

Like different grits of sandpaper give different finishes, big rocks and coarse gravel-sized stuff scratches deep grooves into the underlying bedrock while the small stuff polishes bedrock smooth. It is these grooves that we call glacial striations. Like roche moutonnees point in the direction of glacial flow, so to do glacial striations indicate the direction of flow for the glacier. In Vermont there are three possible directions.

  1. The first is the flow of the continental glacier as they pushed towards the south. This movement entirely overrides any local topography.
  2. The second is regional glacial movement constrained by the north-south trending mountain ranges in Vermont. This flow roughly follows the continental flow so mostly appears parallel.
  3. The third is local movement of valley glaciers. This follows local topography. As the glaciers retreated (melted), small tongues of the ice sheet would have persisted in higher elevations and valleys, slowly grinding away at the bedrock before giving up their stronghold on our mountains as the climate warmed. These valley glaciers leave striations that follow local topography and may be east-west, north-south, etc.

Examples of glacial striations