IMAGE:  December 2002 GRAPHIC:  University of Chicago Magazine
Volume 95, Issue 2
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Reading between the rocks
Susan Kidwell's stratigraphy course gives students a layer-by-layer view of geophysical history.

The students—seven in all, both graduate and undergraduate—drift in for Tuesday morning class in early October and settle at desks like pebbles along a streambed. Some move sluggishly, still eating breakfasts of coffee and muffins; others whirl in briskly and, before plunking down, chat about the quarter's first meeting of the GeoUnion student group.

IMAGE:When not teaching, Susan Kidwell is piecing together the Colorado River delta's history.
Photographs by Susan Kidwell

When not teaching, Susan Kidwell is piecing together the Colorado River delta's history.

Last to arrive is Susan Kidwell, one of the fast movers. Barely five feet tall and wearing a black-and-white plaid flannel shirt, black chinos, and well-worn Birkenstock sandals, the geophysical-sciences professor charges into the Hinds first-floor classroom already speaking: "Finally. We get started." She lands at a spot at the front and keeps talking—about her hectic travel schedule (she'd missed the class's first session because of a late plane flight), about the textbook ("$107! Maybe that doesn't strike you as expensive, but it seems shocking to me"), about the 200-level course in which these aspiring geophysical scientists have enrolled: Principles of Stratigraphy.

First she makes sure the students—who by now have taken Physical Geology, Earth History, and possibly Introduction to Petrology—know their geology. "What is stratigraphy?" she asks and, when nobody volunteers an answer, provides one: "It's the study of layered rocks. So what rocks does that include?"


There's only one book on Susan Kidwell's syllabus for Principles of Stratigraphy: Sedimentary Geology (Freeman) by Donald R. R. Prothero and Fred Schwab. Much of the book won't be covered in class, but Kidwell has no choice; it's the only text on the market that covers stratigraphy. Prothero had an earlier stratigraphy textbook, but "no one taught it but me," says Kidwell.
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"Sedimentary," a shaggy-haired boy in the front replies. Such rocks, as the name implies, are formed by deposits of sediment. Add layer upon layer, and eventually there's stratified rock. Kidwell scrawls the term in cursive on the blackboard. "So that also includes low-grade metamorphics, right? Rocks that began as sedimentary and we can still detect their stratification?" Several heads nod. "Would it include igneous?" Silence on whether rocks formed by the solidification of magma might qualify.

"Do igneous rocks obey the laws of superposition?" Kidwell prompts.

"Some do," offers a woman in a blue-and-orange striped baja and faded jeans. "Ash falls do. And lava flows."

"Does everyone agree?" Kidwell surveys the room. She adds Some Igneous to the chalkboard and turns. "Then are plutonics included?" she demands, referring to igneous rocks such as granite, diorite, and gabbro that solidify and crystallize within the magma chamber.

"Not really," Kidwell again answers her own question. "But because of their cross-cutting relationships with"—that is, a tendency to physically intersect or be intersected by—"sedimentary rocks, they are nonetheless extremely important. They're our best material for radiometric dating. Using them we can import absolute age calls into sequences of otherwise difficult-to-date sedimentary rocks."

And therein lies a hint of what the students will undertake in this course: historical analysis. Stratigraphers, Kidwell explains, are interested in relative dating, always asking, Is this deposit older or younger than that one? She poses another quiz: "So let's say we're looking at a sedimentary rock's contact with a plutonic rock. If the sedimentary rock has a baked contact point, is it older or younger?"

"Older," several students reply at once, and the woman who's helping plan the GeoUnion meeting adds, "It had to be there first to be baked."

"What if the contact point of the sedimentary rock has gravel similar in composition to the plutonic rock?" Kidwell presses. "Younger," the students answer: such a contact point would imply that the plutonic rock had eroded, its gravel becoming an early layer of the younger sedimentary rock. (What Kidwell and her students don't mention is that old rocks can also end up on top of young rocks because of faults associated with the general smash and lift of tectonic forces.)

Satisfied that the students are up to her course's demands ("Good. Now we'll never speak of plutonic rock again"), Kidwell turns to a larger question: why offer a course in stratigraphy? Though it hasn't always been so, she notes, Chicago's is now among the nation's very few freestanding stratigraphy courses. Most geology departments offer courses in sedimentology.

Such courses, she explains, emphasize "facies analysis," or the detailed examination of a sedimentary rock's makeup, aiming to determine, for example, if a sandstone originated as a beach or as the fill in a river or stream channel. From a facies analysis of a rock bed or core sample, sedimentologists diagnose what the environment was like when the rock formed. The driving force behind courses emphasizing facies analysis—and behind the shift away from stratigraphy classes, which until the 1970s were part of the geophysical-sciences canon, she notes—is the desire to find previously organically rich areas in the earth. That is, to find oil.

"I find sedimentology dull," Kidwell declares. "We should all be able to track facies. Stratigraphers step back at least by an arm's length and ask larger questions, ones of correlation, such as, Is this change in the rock driven by environmental change? We reconstruct geological history. We look at the facies, yes, but also at the stacking of stratigraphic arrays, at the sequences of rock and then the supersequences."

And that's why Kidwell, who won a 1999 Quantrell Award for undergraduate teaching, is so concerned that her students know the fundamentals on which arm's-length analyses are based—not only which rocks are which but also how to analyze a rock, determining its composition and texture, the size and shape of its grain using the field's standardized nomenclature, its physical relationship with other bodies of rock around it. Students also must be aware of environmental influences, such as how groundwater flows and how faults and folds affect stratigraphic arrays.

In the weekly lab the class will drill on these topics, learning to draw cross-sections and stratigraphic maps. And because "there's the textbook and then there's the field," and rarely the twain shall meet, there will be a field trip to a quarry in Madison, Wisconsin, for students to test their skills on Cambrian marine sandstones and Ordovician limestones with some unusual "teepee" structures and oolite banks like those forming in the Bahamas today. Students will finish the course with the terms and basic field techniques "down cold," Kidwell predicts, and they will also learn how to do a stratigraphic analysis. "It's as close as you'll get to writing a legal brief without attending law school," she notes. "You'll be a Sherlock Holmes."

"Because of a concern with age," Kidwell continues, "stratigraphers are concerned with the overall quality of the geologic record, particularly where there are gaps. What went on when we have no direct record? What went on, of course, is not recorded in the rock. We have to judge from erosion and other clues." Stratigraphers "read between the lines of the stratigraphic record," seeking a history of sea-level, climate, and ecosystem change and tectonic activity. Increasingly they also try to trace the influence of human evolution on the environment. They puzzle over how to define the environment's "natural" state, given all these influences over vast periods of time.

Human impact on the geologic record has been a particular focus for Kidwell's most recent research; her conclusion that humans have altered the environment for as long as humans have existed garnered her a Science cover last year. [This sentence corrects a previous error that it was a Nature cover. Ed.] The interest spills over into two course requirements, a 15-minute oral presentation and a ten-page paper counting for 20 percent of the grade.

Among other options, students can use their term papers to examine the consequences of human attempts to regulate natural geological processes. "What's caused by humans? What's natural?" she reflects. What are the public-policy and legal aspects? There's more to stratigraphy, it seems, than immediately meets the field-geologist's eye.

Perhaps that's why stratigraphic analysis has been a passion for Kidwell since her undergraduate days at the College of William and Mary in the mid-1970s, around the time when stratigraphy was falling out of vogue. "I'm not afraid to find moldy subjects interesting," she jokes, promising to show the class the 1914 image that inspired her to become a historic geologist. "I couldn't tell people that's what I wanted to do, of course. Everyone thought that was the most boring thing on earth."

The students laugh. That this stuff could be boring doesn't seem to enter their minds.

— S.A.S.



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