15 April 2011

Friday fold: A cold one

Posted by Callan Bentley

Whilst browsing the planet’s surface for Where on (Google) Earth #279 last week, I found this interesting fold expressed on the surface of Grewingk Glacier, in the Kenai Mountains of south-central Alaska. Check it out [Google Map link]:

I’m not quite sure what to call this feature. Certainly the “sediment in ice” pattern (dark in light) suggests a medial moraine, but this structure doesn’t meet the textbook definitions of any kind of moraine I know about. It’s certainly not parallel to the glacier like a medial moraine. The overall pattern is perpendicular; perhaps we should dub it a “transverse moraine.” It appears to be a layer of till that has been distorted by glacial flow (downhill is to the northwest). Glaciers, like rivers, tend to have the highest flow velocities in the middle of the glacier, towards the top. A classic demonstration of this fact was performed in the Swiss Alps in the 1870s and 1880s: a series of stakes was driven into the ice in a horizontal line across the Rhône Glacier, and then it was observed how they moved. The ones in the middle moved fastest; the ones at the edges moved slowest. If this odd “transverse moraine” started out the same way, then it could have been distorted by quicker downstream flow in the middle, and less at the edges.

What catches my eye as a structural geologist are the parasitic folds which “decorate” the main fold. Like we saw in the gooily-deformed shales below the Champlain Thrust Fault, parasitic folds are higher-order folds whose asymmetry can be used to infer shearing kinematics on the folded layer:

Note the similarity in outcrop pattern to the Grewingk Glacier example…

What do you think of that?