Sunday, August 7, 2016

Geo 884: Not Just a Pretty Face

As I predicted in yesterday's post, today's is a bit to the left, and without scale. The cobble in the lower right is the same one my foot was resting on in that photo. Just to the right of that cobble, there are some laminations with cross-cutting features, but they're obscure and poorly focused, so let's look at a crop from yesterday's shot.
Here I've highlighted a couple of the beds that cut across other beds. The principle of superposition tells us that younger beds are on top of older beds, and the principle of cutting tells us the feature doing the cutting-- whether that's bed forms, as here, a fault or a dike-- is younger than the rocks it cuts. The highlighted horizons are cutting the other beds nearby, so those horizons are younger. Since they're younger, we can infer they're on top. Thus stratigraphic "up" is the opposite of the orientation in these two photos. Which is to say (at the risk of being confusing) that "up" is down.

Back to today's photo, just a quick comment: who says rocks aren't pretty? There are some interesting details, but take a moment to enjoy to appreciate the esthetic quality of the exposure.

Here's a crop from above ("below") the cobble:
I'm guessing that the round clast on the upper left was able to roll into place, then draped with subsequent sediment. The bump (keep in mind, stratgraphically, that's a divot) in the right middle looks like a burrow of some sort. And toward the upper right, the tent-like form *may* be a very small flame loading structure. And I'd be remiss in failing to point out that all three of those are informed guesses, with varying degrees of confidence.

Another structure, which puzzles me a little, is visible in both yesterday's and today's shots. (The crop is from yesterday's.)
I *think* that's a dewatering structure; as saturated sediment compacts, the interstitial water can escape along a channel and disrupt the sediments it's passing through. However, I think it's also possible that it might be a burrow instead.

Photo unmodified. July 21, 2016. FlashEarth location.

1 comment:

Hollis said...

thanks! yes, beautiful