Geoscience Reference
In-Depth Information
CHAPTER 7
Why Would a Dinosaur Eat a Rock?
Your Inner Gastrolith
The dark rock stuck out in the pinkish mudstone, looking like an errant raisin tossed
into a strawberry mousse. My curiosity properly piqued, I walked over to pick it up
and held it in my left hand, admiring its fine qualities. It was a black, polished chert,
which is a hard, flint-like mineral. It was beautifully rounded and about half the size
of a baseball, but slightly longer than wide. I bounced it up and down to better feel
its heft. A closer look revealed a few irregularities on its otherwise flawless surface,
littlechattermarkswhereitmusthaveimpactedagainstsomeothersolidobject,such
as another rock.
Somehow I identified with this stone, feeling like a small oddity pasted onto
a homogenous background, with an apparently polished exterior but also bearing
blemishes for all to see. It was the summer of 1983 and I was well into the second
week of geology field camp in northwestern Wyoming. About a hundred other geo-
logystudentsweretheretoo,allofusbeingdirectedbythreegeologyprofessors.Yet
Iwasanearth-science neophyte,havingjustfinishedmyfirstyearofanM.S.-degree
programingeology,butenteringitwithabackgroundinbiologyandart.Iwasalsoa
Midwesterner who had been confined to flat, cornfield-dominated landscapes east of
the Mississippi River for all of my life. So this was my first trip to the western U.S.,
and my first time walking on Mesozoic rocks.
Underneath my feet, the weathered mudstone—streaked with white, gray, or
maroon, and occasionally interrupted by beige sandstone lenses—was the Late Jur-
assic Morrison Formation, renowned worldwide for its massive fossil bones. This
was dinosaur country, and being in it was absolutely thrilling. I had read about these
giant extinct animals all of my life, but at the time my home state of Indiana had
