Sorry I've been gone so long! My kidneys conspired with some bacteria in order to attempt murder upon me, probably because they're tired of me threatening to donate the one that used to get all the kidney stones. I've now recovered about 60% of my brain function, which means I can start talking geology to you again. Alas, since I'm not up to full capacity yet, we'll have to do some easy stuff. You'll enjoy it, though. I mean, who can resist such a happy rock?
If you look closely at the photo, you'll see layers. The layer our happy rock is in is a slightly coarse sandstone. The layers above and below are mudstone, much weaker, flakier, and very fine-grained. This is a classic section of a turbidite. These are basically the result of undersea landslides, called turbidity currents, which can be caused by a variety of things. This may be recording an ancient sequence of earthquakes, a series of storms, or even just gravity working. Something triggers a slurry of sand, mud, and water which flows down a steep slope, coming to rest in a particular sequence. First, the coarse particles settle out, creating a layer of sand. Then the silt-sized particles settle, forming a layer of mudstone.
This turbedite sequence is part of the Tyee Formation up on Marys Peak in Oregon. You can see it from several places along the road. This outcrop is a particularly fun one to explore, as it's still nearly horizontal, and quite tall.
You can get an excellent view of the discrete layers. It's really rather neat, being able to get your hands on ancient underwater chaos.
You can tell the cliff's quite crumbly. This isn't strong stuff - definitely not the type of stone you'd want to build a house out of. There's a considerable amount of loose rock that's eroded off, and that's where you can have some super-awesome fun. Check this out: wee little plant fossil fragments all up in the sandstone!
Alas for fossil enthusiasts, turbidity currents aren't the calm, quiet depositional environments you need for preserving whole leaves and things. This stuff always looks like it's been through a blender. But I still enjoy finding it.
What's even more awesome, though, is when you find a rip-up clast. Like this beauty I discovered at the base of the cliff.
How gorgeous is that? What's happened here is all kind of exciting and dramatic. Remember when I said the silty particles settle out last, and create a nice mud? That becomes the top layer. Then, along comes another turbidity current. It'll rip up bits of mud as it roars across, and incorporate them into the sandy material that's settling out. Voila - rip-up clasts!
This is the first I've found in a hand-sample sized piece of sandstone, so I'm totally stoked. Next time you pass by a cliff that's got alternating sandy and muddy layers that turns out to be a turbidite sequence, and you get a chance to (safely!) stop, see what you can find in the rubble at the base. And just consider, you're holding a pretty dramatic geologic event in your hand!
To give you an idea what a turbidity current's like, here's a video from Western Washington University demonstrating them in a lab setting:
And if you want to see the real thing, here's an ROV getting hit by a turbidity current in a submarine canyon. It's the last half that's the really exciting part, in case you want to skip forward a bit:
Just think: millions of years from now, that turbidity current may be part of a formation being studied by future geologists. Neat!