4 October 2019

Friday fold: Angel Island “IV”

Posted by Callan Bentley

Hey, let’s go back to Angel Island for today’s Friday fold. We’ve been there once, twice, thrice previously.

The rocks in question are metaconglomerates that Jess Ball and I found at first only as float on the beach at Camp Reynolds, like these two examples:

…Look at those beautiful elongated pebbles, transected by wee white veins!

Where there’s float, there may be outcrop — Sure enough, with a minute or two of searching, we found this same beautiful stretched-pebble conglomerate cropping out to the south, in cliffs along the shore.

Now let’s take a little tangent into looking at deformation within the clasts….

Here’s a close up of one of the deformed pebbles:

Note the set of fractures that are more or less perpendicular to the long axis of the thing; this implies both ductile and then brittle deformation of the clast.

More examples of the same phenomenon:

So that’s a lovely bit of deformation in a metaconglomerate, I’m sure we can all agree. But where are the folds, you may ask?

Here’s one spot – a bit of waviness that has an S-C feel to it:

What’s being folded here is the foliation, defined the shape of the deformed pebbles. Here it is, traced out:

And, in a less severe example, there’s a very slight arching to the foliation here, steeper at bottom left, shallower at upper right:

And here’s some slight warping of one of the stretched pebbles:

Note the varying orientation of the X-axis-perpendicular fractures. It almost looks like a segmented grub, undulating across the outcrop surface!

I hope you find these unimpeachably solid folds, even if they are a bit subtle. Hopefully the rest of the structure in the outcrop is icing on the cake.

Happy Friday to you!