Welcome to Module 9

Deep Time: Stratigraphy and the Sedimentary Record

With apologies to the great detective Sherlock Holmes, here is a brief introduction to the science of geological history and multiple working hypotheses, together with some bad Sherlock Holmes jokes. If you don’t know anything about Sherlock Holmes, just note that scientists and detectives can learn what caused events in the past. One way we do this is through “multiple working hypotheses”. After you observe something, you generate as many hypotheses as you can about what produced the thing you observed, figure out what each of the hypotheses predicts about additional things you could observe, and make those observations to see which of your hypotheses comes closest to being correct.

"Holmes, did you realize that this chess board is exactly 12 inches long?" I asked, as we sat across the table in our Baker Street lodgings.

"Come, Watson. The game is a foot," he replied. "We must explain the Giant Splat of Sue Matra's birthday."

"The Giant Splat? Holmes, I know that Sue Matra is a friend of your brother Mycroft, but how came she to have a giant splat on her birthday?"

"Sue returned home from her job as a scientist measuring the air speed of swallows carrying coconuts and discovered a splat on the floor. Here is a brief diagram of the situation."

Holmes rapidly sketched the following:

Sketch of a blue cake, as described in the text, bitten by a dog, flipped upside down.

Sketch of a cake

Credit: Sherlock Holmes

"This takes the cake," I commented sweetly, after studying the diagram.

"No," he replied, "The cake was left on Sue's floor."

"Holmes," I said, "You know that my medical training exposed me to the scientific method and that in my military experience, I conducted forensic investigations, learning what people died of. I was even exposed to geology and paleontology during my studies at the college. Your science of detection shares much with forensic medicine, and with the fields of geology and paleontology. Something has happened, and you try to determine what."

"Indeed, Watson, I am aware that you are among those few individuals who have been trained to reason from effects to causes, so unusual in a world full of people who reason only from causes to effects."

"But it is more than that, Holmes. When faced with an effect, we quickly try to think of all possible causes that are consistent with the available data and with our other knowledge about the world—call these 'multiple working hypotheses'. Then, we see what each of them predicts that we might discover or measure—each possible hypothesis is a possible cause, and we reason to its effects in the same way that other people reason from causes to effects. But we go further, and we make those measurements or observations that will identify the effects of each of our likely causes, using the results to eliminate some of the hypotheses. For example, if we postulate that a horse stomped on the cake to make the giant splat, then we expect the mark of a horseshoe in the cake."

"Even if the horse missed, the shaking of the ground might have caused the cake to fall. Close counts in horseshoes and hand grenades, as you know."

I ignored his attempted witticism, and said, "Holmes, allow me to send a few telegrams to test some hypotheses, and we will soon have the answers."

An hour later, I was ready to explain the giant splat to Holmes. "Sue Matra is married to a diligent and helpful husband, who is also an amateur cook. He confirms that he was baking a birthday cake for Sue, and made it in three layers, putting down a layer of cake, then icing, cake, icing, cake, icing. Then, he put candles in, and carefully carried the cake across the study toward a table. Unfortunately, the couple owns a large and boisterous retrieving dog, which jumped up and took a large bite out of the cake."

"The curious incident of the dog in the daytime," Holmes muttered.

"Yes," I said, "the dog did much in the daytime. You will notice that your highly accurate sketch shows that the dog's bite has severed one candle, and that the candle was shoved through the icing, so the order of events is clear. The actions of the dog so surprised the husband that he dropped the cake, which was unbalanced by the dog anyway. Given the height from which it dropped, and the rotation imparted by the dog, the cake flipped halfway over and splatted to the floor. The husband, distraught, and worried lest the suddenly sweetened canine should have an accident on the floor, took the beast for a walk, and during this interval, Sue returned home and discovered the splat."

"And how," asked Holmes, "did you determine that the dog bite occurred before the splat."

"If the splat occurred first and the dog then bit the splatted cake, the bite would not run smoothly through the whole thickness of cake, or else the dog's teeth would have scarred the floor or become stuck in the wood, yet the bite through the cake runs smoothly the whole way."

"Well," said Holmes, "I hope they have some good old H₂O to clean up the mess."

"Elementary, Holmes," I replied, "elementary."

Could you have explained Sue's splat? If so, you understand some of the secrets of being a geologist. If not, then stick with us, and you should have it figured out by the end of the module.

Video: The Cake (3:31 minutes)

Click here for a transcript of The Cake video.

Dr. Richard B. Alley: It's the weekend, and you're baking a cake for the game for the big game. And so you put down a layer of cake like this. And then it's gotta be for the Penn State game. So then you're going to put in a little blue icing, so it's more enjoyable. And you're going to make another layer cake on top of that, still sitting there. Boy, you're a good baker. It's really very impressive.

And we'll put some more Penn State blue icing in the middle. And then you put down one more layer of cake to make the top of the thing. And wow, this is getting to be fairly serious when all is said and done. And then you're going to ice the top of that, and wow, isn't that beautiful?

And then you want to put some candles on. It's going to be a Wisconsin game, and Wisconsin's going to get burned. So you better put on some Wisconsin red candles, and they have a little wick sitting out the top of them like that. And then you're carrying the cake across the floor and your big dog comes up and takes a gimungous bite out of your cake, which doesn't necessarily make you entirely happy. And so now your cake is going to look something more like this with this big hole out of it. And the hole goes all the way down, something like that.

Oh dear, what a mess the dog made out of the cake. And you're really very, very unhappy about what your dog did to it because you were carrying the cake while this happened, and you happen to know that if you're carrying the cake while it happens that you may lose your grip, and the cake turns upside-down, which is now a real mess, and the candles gets scrunched, and so they're no longer sticking down the way they're supposed to be. They're actually sort of sticking off to the side the way they get bent over.

Now, if you were to see this, if you were to find this cake on the floor, you would have absolutely no trouble telling what happened, and the sequence of events in which they happened. In the same way, a geologist faced with something such as a rock viewed edge on that has some mud cracks in it like this, there's a crack, says oh, this rock is right-side up, it has not been turned over because we know that when mud cracks, that the cracks go down in the rock, not up.

In the same sense, if the geologist was faced with some sort of a footprint, and let's make a dinosaur footprint here if you would like to. Do, de-do, de-do. Here's the big footprint. OK. And the footprint is stuck down in the rock, something like that. And so you can see this is how it might have happened somehow. And that's not the world's finest footprint drawing, but you get the idea what we're trying to do. That has been shoved down into a rock so there's a big hole here where the footprint was made.

If you were to happen to find that footprint, and you're going to have to find that footprint as being upside-down, as we can make it very easily with the foot vertically, you wouldn't know that. And so geologists do that. They can put things in order in the same way you that could look a that squished cake and put it in order.

Learning Objectives

Understand that sedimentary rocks are produced from sediments by well-understood processes.
Explain how characteristics of sediments and sedimentary rocks, and the fossils they contain, reveal the conditions under which they formed.
Be able to interpret sediments and sedimentary rocks to put them in geological order.
Explain how putting sedimentary rocks in order puts their fossil types in order, showing gradual changes over long periods of time.

What to do for Module 9?

You will have one week to complete Module 9. See the course calendar in Canvas for specific due dates.

Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.

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Welcome to Module 9