Geoscience Reference
In-Depth Information
live east of the Mississippi River, however, this depressing situation is the norm.
Despite more than 200 years of paleontological study, Mesozoic rocks of the
eastern U.S. are embarrassingly shy about revealing dinosaur bones, although a
few places—such as in Connecticut, Maryland, Massachusetts, and Virginia—have
plenty of dinosaur tracks. This general lack of dinosaur bones is credited to poor
preservational conditions—not enough dinosaur bodies getting buried quickly and
in anaerobic environments—and what may have happened to bones after burial,
suchasdissolutionbyacidicgroundwater.Yetmanyoftheseenvironmentsmaynot
have been all that good for preserving dinosaur tracks, either. For example, only a
few theropod tracks have been found thus far in Late Triassic rocks of northeast-
ernVirginia,despitethesehavingbeenformedinvalleyswithabundantriverflood-
plains and lakes. Finally, paleontologists and geologists in the eastern U.S. face a
challenge that is not so overt in the arid western U.S. states: plants. This lush ve-
getation not only covers up or otherwise obscures rocks, but also destroys original
bedding and other primary features, such as dinosaur tracks, via root activity and
soil formation, often prompting eastern U.S. geologists to jokingly praise the vir-
tues of defoliants.
Weems and his colleagues built their case about the Bull Run gastroliths by
noting a number of traits that matched those of known dinosaur gastroliths:
• These rocks stuck out as unusual concentrations of pebbles, gravel, and
cobbles weathering out of the Bull Run Formation, most of which is com-
posed of finer-grained strata such as siltstones and sandstones.
• More than two hundred of them were recovered from a small area of about
30 m
2
(323 ft
2
), just slightly more than the size of a baseball diamond.
• Alloftherocksweremadeofquartz.Mostwerequartzite (metamorphosed
sandstone), and the rest originated from quartz veins that formed in either
igneous or metamorphic rocks. No limestones, sandstones, or other sedi-
mentary rocks are represented.
• Nearly all were 2 to 5 cm (<1-2 in) wide, and the biggest was about 10.3
cm (4 in) wide.
