On one of my field trips last week, I collected this cobble of sandstone (penny for scale):

And here's the other side:

There's a delicate but telling geopetal indicator here in this sandstone: it shows us which way these sandstone layers were oriented in space when they were deposited as loose sand. Geopetal indicators give us "paleo-up," sometimes called the "younging direction." Classic examples include graded bedding, cross-bedding, and mudcracks. Here, it's a bit more subtle: small erosional "divots" in the layers of sand. These "divots" may be caused by something pushing down into the sand (the trace of an organism's trail), or may be caused by small amounts of scouring erosion. We only get to see them in two dimensions, so it's unknown whether they are simple pinpoints in three dimensions, or linear features -- perhaps even branching linear features. Reviewing the cobble's many layers, I've found three types of "divots":

Type 1 is a simple deflection of the the dark layers. It is more likely that the layer is deflected downward, but there is no guarantee: it could be a little lump of sand poking up from the bottom, too. In other words, Type 1 is not a completely compelling clue for paleo-up. Type 2 is more convincing as a geopetal indicator: here a lower layer or two has been actively scoured, and then an upper layer is draped over the scoured-out hole. Type 3 can also be seen though, and it's a weird one: I'm having a hard time coming up with a reason why two successive beds would both have a "divot" in the same location. Is this a squishing downward effect? For instance, were I to go stand on my bed, my weight would push downward on my comforter, but also the sheets underneath. They would both deflect from the bed's horizontal surface in the same downward direction. (Would this be a "duvet divot?")

See if you can find examples of all three in the photos above.

Labels: coastal plain, primary structures, sediment