Posting the thing
July 7, 2016 4:08 PM Subscribe
Fun fact 1: Bentonite, a mineral formed when volcanic ash falls into the sea and gets deposited, separates out the fencepost layer of limestone from the rest of the Greenhorn Limestone formation and makes it easier to separate and quarry from surrounding rocks. So those fenceposts are partially due to giant volcanoes erupting much farther west, in either the NW or in the SW as part of the Sevier orogeny (mountain building event) in Utah and Nevada. This event caused what's known as crustal shortening (one crust is thrust over another, shortening the crust; it's the opposite of rifting, when the crust extends is stretched apart), of which some of the evidence for can be found in minerals in. . .the Greenhorn Limestone! That's a stress transfer distance of over 1200 miles! That's just neat!
Fun fact 2: you can tell if the limestone in a fencepost was from the so-called Fencepost layer of the Cretaceous Greenhorn limestone or from some other layer (or a harder, less clay rich layer of the Niobrara chalk such as the Fort Hays member) if it has a streak of brown in it, a mineral called limonite which is an iron mineral. You can find all kinds of things in this "streak" including cubes of limonite which has replaced pyrite. And driftwood! It's a weird layer and signals a change in the late Cretaceous western seaway, a change which will lead to one of the greatest chalks ever deposited, the Niobrara chalk.
The age of these "fenceposts" is generally Turonian in age (early late Cretaceous, between 89-94 million years old). Look on the back of them for a clam called Inoceramus. It was a period of very very slow deposition with pretty small deposition rates and low oxygenation, and really high sea level.
I've spent hundreds of hours working in the Greenhorn limestone and my heart always quickens a beat when I top a rise in western Kansas and see limestone fence posts, because I know I'm going to be working on it again.
posted by barchan at 5:20 PM on July 7, 2016 [19 favorites]
Fun fact 2: you can tell if the limestone in a fencepost was from the so-called Fencepost layer of the Cretaceous Greenhorn limestone or from some other layer (or a harder, less clay rich layer of the Niobrara chalk such as the Fort Hays member) if it has a streak of brown in it, a mineral called limonite which is an iron mineral. You can find all kinds of things in this "streak" including cubes of limonite which has replaced pyrite. And driftwood! It's a weird layer and signals a change in the late Cretaceous western seaway, a change which will lead to one of the greatest chalks ever deposited, the Niobrara chalk.
The age of these "fenceposts" is generally Turonian in age (early late Cretaceous, between 89-94 million years old). Look on the back of them for a clam called Inoceramus. It was a period of very very slow deposition with pretty small deposition rates and low oxygenation, and really high sea level.
I've spent hundreds of hours working in the Greenhorn limestone and my heart always quickens a beat when I top a rise in western Kansas and see limestone fence posts, because I know I'm going to be working on it again.
posted by barchan at 5:20 PM on July 7, 2016 [19 favorites]
The Fencepost Error is interesting; I never knew it had an accepted name.
I was doing a CAD "fencepost" layout this week that started and ended in the middle of a span (no posts at the ends). It was not immediately obvious that in that case the number of spans and posts was equal.
posted by Mei's lost sandal at 5:46 PM on July 7, 2016
I was doing a CAD "fencepost" layout this week that started and ended in the middle of a span (no posts at the ends). It was not immediately obvious that in that case the number of spans and posts was equal.
posted by Mei's lost sandal at 5:46 PM on July 7, 2016
Ah, barchan, before the end of the first sentence, I knew that was your comment. I'm an environmental geographer, so I just dabble in this stuff, but I love a good geology discussion.
posted by mollweide at 6:14 PM on July 7, 2016 [1 favorite]
posted by mollweide at 6:14 PM on July 7, 2016 [1 favorite]
one of the greatest chalks ever deposited
I love that there are people who can have opinions on the quality of chalk deposition, and have a mental ranking.
posted by Joe in Australia at 7:18 PM on July 7, 2016 [3 favorites]
I love that there are people who can have opinions on the quality of chalk deposition, and have a mental ranking.
posted by Joe in Australia at 7:18 PM on July 7, 2016 [3 favorites]
That last link seems like the medieval equivalent of Buffalo buffalo buffaloing.
posted by jacquilynne at 7:20 PM on July 7, 2016
posted by jacquilynne at 7:20 PM on July 7, 2016
Now I'll need to work in Vitruvius somehow in my next code review.
posted by Harald74 at 11:05 PM on July 7, 2016 [2 favorites]
posted by Harald74 at 11:05 PM on July 7, 2016 [2 favorites]
I never knew what Rough Ashlar's user name meant until I read the fencepost rocks link. Kind of cool when you make a connection like that.
posted by V'Ger at 8:49 AM on July 8, 2016
posted by V'Ger at 8:49 AM on July 8, 2016
The Plural of Fent (my title, not his) by B. Kliban
posted by kurumi at 10:09 AM on July 8, 2016 [1 favorite]
posted by kurumi at 10:09 AM on July 8, 2016 [1 favorite]
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