28 July 2023

Friday fold/fauxld duet from Glacier National Park

Posted by Callan Bentley

Here are a couple of the scenes I was lucky to view last week in Montana’s Glacier National Park:

I like how these two scenes compare and contrast with one another. One shows an anticline; the other a false anticline.

Let’s deal with the faker first: the first image is a scene on the Siyeh Pass trail, which shows almost uniformly dipping strata of the Empire Formation (there are a couple of kink bands). The appearance of a “fold” comes from the varying orientation of the trace of those beds, determined by the orientation of the outcrop surface. Here, a big scoop has been taken out of the mountain by a tributary glacier, and that causes the trace of bedding (yellow, in the annotated copy below) to varying on the cirque’s south, west, and north sides. I’ve highlighted similar “false folds” previously, as with this example from Death Valley.

In contrast, at Two Medicine, an actual anticline is bisected by a valley between Rising Wolf Mountain (left/west) and Spot Mountain (right/east):

This is right on the (eastern) leading edge of the Lewis Thrust sheet, and there’s a lot of deformation there, both in terms of folding of various styles and wavelengths, but also subsidiary thrust faults.

I really love leading this “Geology of Glacier National Park” field course for science teachers every summer; it’s a highlight of my year.