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Archive for May, 2010

It is the east side of Red Canyon that gives it its name. The curious fact is that the west side is completely different. The soil is not particularly red, maybe a little in spots, and the land sweeps away from the canyon road in a gentle slope with just a few dry cliffs because the soil is so weak.

This is the west side of the road, not red at all, and belonging to a much earlier era.

How did this happen?

First of all, we need to look a little farther west, at the Wind River Mountains. These were built – that is, they came up from the earth – at the end of the Cretaceous. The Cretaceous is the third period of the Mesozoic Era (Triassic, Jurassic, Cretaceous). Now, to review: the red shales and siltstones date from the Triassic, and the pink sandstones from the Jurassic, and both were laid down flat, as sediments must be; you remember that Nicholas Steno explained this way back in the 17th century.

But then, in the Cretaceous, the third part of the Mesozoic, something completely different happened. The Wind River Mountains were born; they pushed upwards from below the landscape and tilted everything sideways. They lie just west of our canyon, and when they rose out of the earth, it was crumple time for everything. The sandstones held out best, but whenever they broke up and gave way, the silt and shale followed quickly, and whatever rivers formed on the new landscape took away all the debris, leaving what was under the Triassic soils.

Well, what’s under the Triassic is the Permian, the last period of the Paloezoic.

The Permian Phosphoria formation has some sandstones that are windblown (not river borne) and some limestones, which is to say underwater shell deposits, and some dolomites and other things. None of it is particularly red; it’s mostly white and gray, rather drab, a nice place to rest your eyes from the intensity across the road. So at the bottom of the canyon is Red Canyon Creek, which carried away the soft stuff; then on one side the Permian Phosphoria slopes away to the west, and on the other, the bright Chugwater leaps up towards the Jurassic Nugget on the eastern horizon. It’s two different worlds, dating from millions and millions of years apart, with a little creek and a little red road running between, and all sorts of gray-green sages and things that must notice the difference between the soils, but they just grow along as if time and soils made no difference at all.

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Look into the deep background of yesterday’s photo of Red Canyon, and let your eyes find the lighter pink or salmon-colored Jurassic Nugget Sandstone that forms a rim on the far side of the canyon. Sandstone is harder than siltstone or shale, and does not so easily erode. Once it breaks down, however, the softer redbeds are quickly eroded as well, so that you see the pink cliffs of the Jurassic Nugget, and then, falling steeply below them, the red cliffs of the Chugwater formation. If you drive through Red Canyon, many vistas will juxtapose the two formations, like this image, so you can compare them.

The sandstones of the Jurassic are harder and make stronger cliffs than the red shales of the Triassic Chugwater below them.

Click to enlarge the image and notice how the sandstone cliffs have substantial faces, whereas the shales are mere slides with little shelves of moderately firm siltstone. Siltstone is just what the name suggests – stone made of silt. It’s not very strong.

There are other places where the Chugwater formation actually has sandstone members as well, but here it is only shale and siltstone. The lighter sandstone is quite distinct.

Say: Triassic, Jurassic, Cretaceous.

The three major periods of the Mesozoic Era are the Triassic, the Jurassic, and the Cretaceous. The redbeds here are Triassic, all formed when the earth had a single landmass; the pink sandstones above them are Jurassic Nugget Sandstone.

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Red Canyon WY

A few miles down the road from the ranch site where Wyoming Catholic is starting its campus is an extraordinarily scenic spot called Red Canyon. In fact, if you have time, you can drive into the canyon along the reddest road you are ever likely to see.

The redbeds of the Triassic Era are beautifully displayed in this canyon by the Wind River Range in Wyoming

These red formations are siltstones and shales from the Triassic – about a quarter of a million years ago; that’s 250 – 200 million years ago. In those days, all the continents of the earth were gathered into a single continent which we call Pangaea. The important thing to understand about such a large landmass is that there must always be widespread deserts when so much land is far from the ocean. Even now, the continents of Africa, Australia, Asia, and South America have major deserts because moist ocean winds do not fully refresh their interiors. North America also has deserts, but smaller ones, mainly because humid air from the Gulf of Mexico is drawn north across the continent before it turns east with the prevailing winds; in this way it brings moisture to large parts of the continental center.

But in a single continent, there can be no Gulf to fix things, and thus there developed the most striking characteristic of the Triassic era: worldwide formations brilliant with intensely red-staining iron oxides.

Here in Wyoming, the redbeds are called the Chugwater formation and consist of soft red shales, harder red siltstones, and very strong red sandstones. In the middle distance of the photograph, you can see some shelves set into the cliffside; these are the sandstones and siltstones which do not erode so easily as the shales.

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Sorry, the Gros Ventre Slide was 50 million cubic yards, not 25. What’s that? Do you have any idea what 50 million cubic yards means?

Big.

Yes, and elephants are big too, but they’re not 50 million cubic yards, so let’s get a grip on this particular size of big. Otherwise it’s just mumbling, right?

Well, consider this: 1760 is the number of yards in one mile, and 1760 x 1760 is 3 million. So one square mile is 3 million square yards. One square mile one yard high would be 3 million cubic yards. (Actually, it would be 3,097,600 cubic yards, but never mind the little stuff; it’s not like anyone was actually counting cubic yards, right?)  Since 3 x 16 = 48, we can say that 3 million cubic yards x 16 would make 48 million cubic yards, and, again, that is close enough.

So: rocks covering one square mile of land piled 16 yards high would be close to 50 million cubic yards of material. In South Dakota, you have a pretty good idea of one square mile, because country roads are generally one mile apart. And 16 yards high is 48 feet, which is something like a five-story house or maybe a silo twice as high as the farmhouse. So now you are picturing a section of sloping land completely covered with tall silos that all slide down at once into a little valley, and in fact slide so fast that they start up the hill on the opposide side of the valley before they stop. Would that plug the river in the little valley?

You bet it would. You can read about it at the Ultimate Wyoming web site, here. It’s quite a story.

Since the Gros Ventre Slide was not a square piece of land, it might be better to picture 2 miles long by half a mile wide by 16 yards deep. Or even three miles long and 1/3 mile wide and 16 yards deep. You can find a useful image in Wikipedia, here.

Anyway, that gives you some different ways of thinking, instead of mumbling, about 48-50 million cubic yards.

And here’s a close-up of the hands holding the Gros Ventre Shale, which looks quite solid except where it’s wet and then it’s gooey like clay.

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The Gros Ventre shale, the green stuff near the Ordovician Bighorn Dolomite, takes its name, apparently, from a fat bellied (gros ventre is fat belly) French fur trader who worked a river elsewhere in Wyoming.  The shale itself is from the Cambrian era, which is the beginning, not of life itself, but of life with shells and teeth. It’s just over half a billion years ago, so it’s older than the Ordovician dolomite.

And it’s soft. It’s like a clay when it’s wet. Look at her right thumb.

You can see (below) how the shale lies sideways here, and even a little helter-skelter, but shales are always laid down flat – by gravity; so once upon a time, these shale layers were flat.

Farther west, near Jackson Hole, this shale was recently (geologic recently) involved in a massive landslide. It was 1925, and after a period of very heavy rain, a section of this shale, which was arranged on a steep slope, got so waterlogged and slippery that it gave way and slid down Sheep Mountain, along with the sandstone that was lying on top of it. About 25 million cubic yards of material came crashing down. You are glad you were not canoeing on the river right then. It made a dam on the river and a lake as well. A few years later, part of the dam broke, and another catastrophic flood buried the town down the river.

Where did the shale come from? How did it start out?

Shale is always a deposit at the bottom of a quiet lake or sea. In ancient times, there was a broad seaway in Wyoming, an arm of the Pacific that reached from southern California  all the way up into Idaho. Slow-moving rivers from the east brought mud into the quiet sea-bottom and laid it down gently. Little trilobites in the shale help us identify the seascape.

What sort of massive chaos turned everything up on its side? Probably the building of the Teton mountains nearby. Wyoming has had a lot of geologic turmoil. Well, so have other places, but sometimes the turmoil is buried or the evidence carried away. In Wyoming, it’s right on the surface.

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Joab’s Karst

Here’s your dolomite Bible story.

Karst is limestone landscape. On the surface, limestone may have soil and vegetation like any other rocky surface, but below, the bedrock is soluble in water, particularly in cold water. Whatever cracks develop in limestone – cracks due to a little earthquake or to much smaller disturbances – these cracks collect water and carry it about, and the water dissolves the stone wherever it flows. As a result, caves, creeks, crawl spaces, chimneys, and sinkholes develop.

There is karst under Jerusalem; it is a limestone landscape. And in the time of David, 1000 years BC, when the Jebusites held the city and taunted the Israelites that they could never take it over, David issued a famous challenge to his men. If anyone could get into the city via its wells, he would be made captain of the army. (2 Samuel 5:6 or 1 Chronicles 11:6.)

Joab took up the challenge. There were underground passages from at least two places outside the city, leading to the main water supply under the city and within the walls. These passages were not wide, and they were not man-made so they may not have been well-known to the Jebusites, but they were large enough – or took only a little more widening, to be used by Joab’s men.

How did he know about them? We really do not know, but anyone with sons must realize that little boys know about caves in the neighborhood, and it would have needed only a few young men in Joab’s army – or Joab himself – to recognize the possibilities.

If you have ever been in a limestone cave, you know that these places are full of surprises and little passages that may or may not go anywhere, and that may lead to drops of many feet. Under the Jerusalem, there was one place where a cave system led down a gentle slope to a drop of 37 feet. Someone in ancient times put steps into the gentle part of the passage and the shaft was straight enough so a man could lower a bucket into the water below and pull it right up.

Some sealing was later done done to keep the shaft from plunging any deeper, but basically, it didn’t go deeper because it was dolomite below the limestone, and, as I said yesterday, dolomite is magnesium limestone and it just doesn’t dissolve so easily.

So Joab and his men were able to climb through the caves, shimmy up the 37 foot shaft because it was tight enough — like a chimney — and then they could have hidden in the sloping passage until everyone was up. By the time the Jebusites realized what was going on, it was too late to defend themselves; the enemy was within and without as well.

There are some good drawings and photographs of the whole layout in Galyn’s Israel Photos, and halfway down the page, you can see someone looking up the shaft that Joab once climbed in the dark. I first read about this in Biblical Archeology, but the science section of the New York Times has a story based on that article and posted online.

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Seems I should talk about apple blossoms since I spoke of lilacs, but I’ve just been to Wyoming again, so I have to talk about that.

Shortly after coming into Wyoming from the east, along Interstate 90, you come to a very striking roadcut where deposits from several different periods of time are all smashed together.

Le voici: Actually, this picture was taken looking back towards the east. Isn’t it amazing!

In the foreground, besides the road and some grass, you see (starting at the left) a grey-green clay slope, and then several other colors — mauve, orange, white and peach  as you move deeper into the photo. All these sedimentary rocks are cut by a ribbon of grass slanting from the upper left to the center right; behind that ribbon stands the Bighorn dolomite. Books always describe this dolomite as white; I would call it light orange, and I had to read the captions very carefully for a long time to be sure I was correctly identifying it. Fortunately, captions were not far to seek; there’s one by the side of the road! Perhaps if I had broken out a piece I would have seen a different color. Surface weathering and staining can be confusing.

Dolomite is a slightly mysterious rock. Common limestone is simple — it’s the calcium carbonate rock of years and years of shells falling to the sea bottom and gluing themselves together. Sometimes, for reasons that don’t seem to be clear, limestone gets a strong mix of magnesium — about half calcium carbonate and half magnesium carbonate. This is dolomite. Where does the magnesium come from? Why does it intrude on the limestone? Saltwater seems to help. Heat seems to help. But it’s just not clear exactly how such a substantial and regular build-up of calcium-magnesium carbonate is composed. A geologic mystery!

Dolomite differs from limestone in an interesting way — it does not dissolve so easily. If you have two layers of rock, one limestone and one dolomite, the limestone will dissolve into caves and passages and underground rivercourses, but the dolomite will resist the water and remain a solid rock formation. This can have interesting consequences. Someday, I’ll tell you a Bible story about limestone and dolomite.

In any case, these great dolomite formations are part of the evidence that once, long ago, Wyoming lay under a salty sea, and for a very long time.

How long ago?

Oh, half a billion years or so. Ordovician.

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