Rattails

Dark gray = harder deposits/minerals embeded in the bedrock (like quartz crystals). Light gray are striations, light brown is the softer bedrock,

Okay – so try as I might, I couldn’t track down any rattails, which was supposed to be 6/6 for evidence of advancing glaciers. There are some up on the exposed bedrock where Rte crosses the Lamoille River, but I couldn’t locate those (pdf). So instead of some nice photos you get a terrible Microsoft Paint rendition of a rattail. Rattails form when there’s a hard mineral embedded in the bedrock (like quartz in a limestone). As the glacier grinds down the softer bedrock (creating erratics, glacial polish, and till in the process) it grinds down the softer parts of the rock first and the harder parts remain. These create a sort of defensive barrier against erosion from stuff on the lee side (down glacier) of the harder chunk. The result is a knob with a “tail” of sorts that points in the direction of flow of the glacier.  If we could look at the glacier before it melted away, there would be a gouge on the contact surface from the harder material. Here’s a key to the image above:

  • Light gray: glacial striations
  • Brown: softer bedrock material, raised wedge is the rat tail, the area protected from the erosive powers of the glacier by the harder material
  • Dark gray: harder material embedded in bedrock

Recap of Evidence of Advancing Glaciers

And voila! That’s it. All the evidence left behind by the advancing glaciers. Here are the 6 that I covered:

  1. Roche Moutonnees + Gentle landscapes: Glaciers carved down sharp mountains to rolling hills, forming shallow slopes on stoll side and sharp cliffs on lee side
  2. Glacial striations: Grooves etched in bedrock aligned with movement of the glacier
  3. Glacial polish: Surface of bedrock polished smooth by flowing glacier
  4. Glacial till: Layer of dense unconsolidated material pasted down on top of a glacier
  5. Glacial erratics: Fractured pieces of bedrock displaced from source, vary in size from tiny to large
  6. Rattails: Erosion-resistant grain in bedrock gouges into bedrock and prevents it from eroded material down stream

Gallery of Evidence

More on the topic

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