Evidence for advancing glaciers

Dark gray = harder deposits/minerals embedded 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 rat tails. And I definitely looked (and am still looking). They’re pretty small features and can get covered with soil or erode away pretty easily. There are some up on the exposed bedrock where Rte 2 crosses the Lamoille River that Stephen Wright showed me back in 2008, but I couldn’t locate those when I recently checked (he said he’s had trouble finding them again as well; here’s his description of those: pdf). So instead of some nice photos you get a terrible Microsoft Paint rendition of a rat tail. Rat tails 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