Reading between the rocks
Susan Kidwell's stratigraphy
course gives students a layer-by-layer view of geophysical
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.
by Susan Kidwell
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?"
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.
"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
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
"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.