Quick-Tour of the Images and Results
It's really hard to visualize the geological column, and the immense ages of different rocks. Yet, it's easy to find geological maps that present this information as if we know all about it. It's also really hard to work from fossil data, and arrive at a sense of which geological strata are which.
This particular investigation addresses only the relatively simple question of "What's under the surface?" It uses eastern Montana as the locale, in part because this is one of the places where it is possible to find really beautiful ammonite fossils. Most people don't know about ammonites, but do know about trilobites--and, given the difficulties understanding "deep time," don't necessarily understand that they lived at very different times.
What to do
1. print out the several pdf files.
2. cut out the core samples and the image of microfossils, and put them in envelopes so they won't get lost as easily.
3. collate one map, one graph of elevation vs location, and one envelope for each group of students.
4. save the other images for your own reference--and get a geological map of Montana to show that all this stuff is real! I like the Geological Highway Maps of regions of the US because they cover a larger area than single-state maps.
I enjoy presenting this as if I'm planning a summer vacation. I'm going to Montana because I've heard that there are some wonderful fossils there. I've heard that there are dinosaurs and dinosaur eggs, but what I'd really like is trilobites. What I'd like to know is how far down I might have to dig to find trilobites.
[Now, of course, what I'd really do is get a geological map and figure this out. But we want our students to get a sense of how we know what's on those maps. So, I choose a different route.]
During the years of extensive exploration for oil in the US--days that are essentially over because we know where nearly all of it is worldwide--the different oil companies drilled "cores." They drilled into the earth with a hollow drill, and brought up the core of rock that was inside the hollow center of the drill. A typical core might be 8-10 inches in diameter and up to 10- or 20,000 feet long. From the rock types and micro-fossils in the rocks, the oil geologists could determine whether the rocks were the right type or the right age to contain oil.
In the last decade or so, the oil companies have donated their core samples to geology libraries at many universities, where they can be examined by the public.
So, I looked up some reports of what had been found. One of the reports, USGS Digital Data Series 57 (DDS57) describes the upper layers of eastern Montana. This report alone examines some 8000 core samples. [The number is important--we're talking huge datasets. DDS14 reports on some 23,000 radiometric dates from cores or other samples, which are only half of the reports known at that time. Many more have since been published.] One of the pdf files is a map of Montana, showing the places where some of the cores were taken--an east/west "transect" from the North Dakota state line to the front range of the Rockies. Another of the pdf's is a graph that shows the elevation above sea level along this transect--with the line indicating the surface. The arrows indicate where the cores were taken. I've also marked on this map the rocks that are exposed at the surface, using the standard geological abbreviations (Km = middle Cretaceous, Ku = upper Cretaceous, etc).
Included are the "steps in the logic" that I used. I took the information from the report on the core samples and color-coded it to make it easy to find my way around (the core data provided the vertical boundaries and the thickness of strata from each age). Then I lined up the cores with the surface diagram, and connected the corresponding regions of the cores. The shorter cores are from DDS57; the longer ones are from my Geological Highway Map.
Of course, rocks don't come color-coded according to age. To make this more realistic, I identified a number of microfossils that are found at various ages, and identified where they would be in these cores. Then, I drew the cores as they might actually look, with different shading patterns for different rock types (sandstone, shale, limestone, granite, etc). The information for students, then, is the pictures of the cores as 'columns of rocks' with the fossil zones identified. Then, to facilitate the analysis of the cores, I cut them apart from each other and put them in an envelope.
So, for the students, each of the cores from these regions is illustrated in the envelope of samples. The top of each core is the surface. On each core, the patterns of light and dark indicate different rock types. The brackets indicate where particular microfossils were found in these cores. There are lots of microfossils; the few that are shown with the core samples are shown in a figure included with the cores. Some of the cores are from one analysis that only looked at the top layers. Others are from a different analysis that looked much farther down.
What I need to do now is figure out how far down each type of rock is. We should be able to correlate the different rocks from core to core, and align them with the fossils as well. Let's do that, and see if we can make a map of what's below the surface?
Here, what I'm trying to get at is using the data to construct our knowledge. Geological maps are not made up; they are based on lots and lots of data. Remember, DDS57 reports on 8000 cores!
We learn, for example, that Jurassic rocks that are exposed in the Mississippi River bluffs (remember that dinosaur skeleton described in the Lewis and Clark journal?) are the same geological layer as the Jurassic rocks that are exposed at the base of the Front Range. We correlate these different surface rocks not just from having similar dinosaur fossils, but because we can show that they are, in fact, the same strata.
We learn that in all of these kinds of cores, the microfossils always show up in the same relative order. Sometimes (as in one or more cores here) strata are "missing," presumably because that particular region was exposed on the surface for some time, and those strata were eroded away. But, the strata that are not missing, above and below, nonetheless allow us to correlate the strata. This is interesting--the strata alone would tell us which layers correlate with which others, simply from the pattern of rock types; similarly, the sequence of fossils alone would tell us which layers correlate with which others. Together, these two types of data make a very compelling case for older strata being deposited first, and younger strata being deposited on top of older strata, with a concomitant change in the life forms that were present at those times.
I would want to couple this analysis with a bit of student exploration on the web. For one thing, we haven't yet answered my question: how far down do we have to dig to find trilobites? Given that we are working with microfossils here, it will be extraordinarily difficult for students to find these particular microfossils by comparing images from site to site (yuck!). However, one of the pdf files shows these and some other fossils against the color-coded map that identifies the names of the ages. From this, they can see that most of the Mesozoic rocks are represented by microfossils, the Carboniferous is represented, and the Devonian and Silurian are represented. In my experience, trilobites are relatively abundant in the Silurian, hard to find in the Carboniferous, and of course gone after the Permian Extinction. So, we'd need to dig a long way to get to the Silurian. To reach this conclusion (or even the more general conclusion that we need to look in rocks between Cambrian and Permian ages), students will need to find some information about trilobites, and what strata they were found in. There's lots of this kind of information out there. Once we tell them where the microfossils are found relative to the names of the ages, they can probably proceed fairly easily.
I would also want to conjure up an image--maybe even do a drawing or a poster--of What Life Was Like at a particular age. They may all want to do Jurassic or Cretaceous because of the dinosaurs, but some of the older ages might be even more fun. Let's imagine the Devonian. What fossils have been found from Devonian strata? There's not much in rocks that formed in terrestrial environments (volcanic ash falls, river banks, etc). Life was mostly restricted to the oceans. There, there were bazillions of fish, from somewhat familiar-looking sharks to weirdo fish covered with big horny shields, with beaks rather than teeth that we'd recognize now. The point is that there just are no fossils from this time period of anything living now. Where did these Devonian creatures go? Where did the current things come from? If life was all in the oceans, how did living things invade the land? We can to museums and see dioramas of "artists' conceptions" of different ages...how did they decide what to put into those dioramas? Quite simply, they put in only those things for which there are fossils from that age.
I think that this sort of strategy will go farther toward giving students an understanding of "The Fossil Record" and what it actually tells us than will our normal strategy of saying "fossils are evidence for evolution." Gee...Aristotle knew about fossils, but they didn't tell him about evolution. He figured they were trapped stars. So, rather than present the conclusion, I think it might be great fun to give students some real data, and then have them work from there and construct their own understanding of what the data mean.