Geo 730: Jan. 27, Day 392: Terraced Roadcut

Looking across 199, Dana shot this photo of a roadcut leading up (or down, it looks like) to the tunnel. If the strike and dip of the Galice on that side of the road is similar to that of this side (which is by no means assured), what might look like near-horizontal bedding is actually a happenstance of the slope of the cut intersecting with the slate's foliation at an orientation that is close to strike. However, the eye-catching thing to me here is the pair of pronounced terraces, or flat berms, cut into the hillside in the middle and upper portion of the slope. This is a common practice in large cuts, and is engineered for two reasons that I know of. First, it forces removal of more mass from higher points, and thus helps prevent oversteepening, decreasing the risk of slope failure. Second, as rock weathers and breaks up, some amount of rockfalls and raveling are inevitable. The flat berms help capture a lot of that mass movement, and prevent it from reaching and obstructing the roadway.

Photo by Dana Hunter, unmodified. May 8, 2013. FlashEarth Location. Indexed.

Sunday Funnies: Play It Cool Edition

Bits and Pieces

Funny to Me

Bits and Pieces

Texts from TNG

The Awkward Yeti

Bad Newspaper

Are You Talking to Meme?

Happle Tea

"Trying to eat breakfast the morning of my defense" What Should We Call Grad School?

Gunshow

xkcd

Sober in a Nightclub

Sober in a Nightclub

Bits and Pieces

Tastefully Offensive

Beatrice the Biologist

Cheezburger

Savage Chickens

Journal of Are You Fucking Kidding!?

Tree Lobsters

Seismogenic Zone

Berkley Mews

Are You Talking to Meme?

Bizarro There are five comics at this link; the above is my favorite, but they're all pretty funny.

Are You Talking to Meme?

Darius Whiteplume

Jim Benton

Cheezburger

Cyanide and Happiness

"My blueberry muffin looks exactly like my dog." Sofa Pizza

Matt Bors

Funny to Me

Funny to Me

See Mike Draw

Texts from TNG

Bits and Pieces

Tastefully Offensive

Savage Chickens

Wondermark

Darius Whiteplume

Julia Segal

Texts from TNG

Bad Newspaper They'll get it out one of these days!

Spud Comics

Sober in a Nightclub

Cyanide and Happiness

Owl Turd Comix

Blackadder

Geo 730: Jan. 26, Day 391: Iridescent Weathering

I can't remember if I've stopped at this particular spot before, but I'm pretty sure this is a feature of the Galice Slate that's new to me- though a phenomenon that I've seen many, many times. Note the bright, highly variable, metallic colors on the cleavage surface of this stone (They are more apparent in the full-size photo). There is a spot along 199 where one can see black smoker sulfides intermingled with altered pillow basalts and breccias (I didn't spot it on this trip, which troubled and puzzled me; it's an easy spot to find, with some good landmarks), but this location is well up-section of that one, and I doubt that's what we're seeing here.

The key clue here is the presence of small reddish-brown halos and spots- there are two larger ones in the lower left middle of the face above, plus maybe a half dozen smaller ones than can be picked out at the scale displayed, and many more in the full-sized photo. My suspicion is that these are secondary pyrite crystals, likely with other metallic solid substitutions for the Fe+2, formed in a reducing sedimentary environment. After lithification, metamorphosis, exposure to surface conditions, and weathering, this pyrite oxidized, and the metal oxides and hydroxides and were distributed across available joint surfaces, including those along foliation.

I can't be certain of this hypothesis, of course, but as I say, I've seen similar "rainbow coloring" in many cases where the fresh rock contains a small to fair amount of pyrite, but which has lost its pyrite in weathered portions. The sulfur is lost as SO2, but the metallic ions have limited mobilities, and tend to precipitate on surfaces that are somewhat permeable to water, fairly close to the iron-stained spots that indicate where the original pyrite crystals have been lost.

Photo unmodified. May 8, 2013. FlashEarth Location. Indexed.