Tuesday, February 12, 2008

Chert

When I first drove to Missouri from New Orleans, I pulled over around Springfield to take pictures of limestone bluffs, all dripping with icicles. I was excited about being in a new place, having come from the alluvial, geologically simple Lower Mississippi Alluvial Basin. As I drove deeper into the Ozarks, I stopped again to take pictures of dolomite outcroppings, ever impressed with roadside geology. Of course, these pictures are long gone, replaced with stellar shots from natural settings where the rock layers are covered in grasses, ferns, and mosses rather than represented in road cuts.

On my first canoe trip on the Niangua River, I recall laughing heartily, with great glee, upon seeing an entire riverbed filled with beautifully colored, interesting rocks. I collected so many rocks that by the end of the 12 mile trip, my colleague had to sit down and reason with me: “Allison…it’s just chert…it’s everywhere in the Ozarks…you really don’t need to take it home with you…it’s just chert...think of the crawfish who live under that chert.” But chert, unlike dolomite and limestone, is pink, blue, green, red and white striped! Every rock is different, all with interesting patterns, many of them with fossils. I inconspicuously packed about 8 big pieces of chert into my colleagues’ car. When I returned to my cabin, I carefully lined them up on my bookshelf, which was otherwise empty but for the Flora of Missouri and a big chunk of bright red rhyolite I picked up in a parking lot.

Chert is usually a sedimentary rock, but can, on occasion, be formed in igneous rock. It’s a silica (SiO2) and calcium silicate rock that measures 7 on the Mohs hardness scale. In other parts of the Midwest, chert is called flint. Because of its hardness and abundance in the Ozarks, chert was used presettlement in the creation of arrowheads.

It formed in one of three ways, all occurring when the Ozarks were underwater: first, silica’s solid particles chemically separated from water molecules to form a soft nodule, which was then embedded in layers of limestone and sandstone. Second, silica was incorporated in soft bodied organisms like algae and cyanobacteria which formed into colonies, or reef-like structures (called stromatolites). As these were compressed, they turned to rock, which was then embedded in limestone. (Fossils of reef structures are pretty common in chert.) Finally, chert was laid down during the cooling of volcanic lava events, where it formed a thick layer on the seafloor below the depth of limestone formation. So, now, when limestone and dolomite erode, previously embedded chert is left behind. This, of course, explains why chert pebbles are so common on glades, for starters.

Precambrian fossils of algae (which may or may not have depended on photosynthesis) have been found in chert. Cambrian Period fossils of plated, chitonous mollusks representing an extinct class have been found in chert. As late as the Ordovician Period, chert fossil records reveal the higher order of cephalopods. In Missouri’s Ozarks, in areas where limestone and chert prevail, you can pick up 20 rocks and find fossils in at least one of them. I’ve just combed through my fern and algae fossil collection, which is lined up on my bookshelf. I can’t attach a picture tonight, because my fossil collection is all limestone-based. My big chert rocks from the Niangua went back to the Niangua on a later canoe trip because I had great hopes I would end up in the Ozarks, where chert-- lovely, dynamic chert --is everywhere.

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