Geoscience Reference
In-Depth Information
members to mold and demold these large nests. The day the molds
were removed, much of the crew had to help; hours of steady pulling
on the latex were required to free the molds from the nests, but by the
end of the season, we had two enormous burrito-like silicon wraps to
take back home for further analysis.
While many of us were busy mapping eggs and molding nests at egg
layer 4, our geologists David Loope and Jim Schmitt, who works
with Frankie at Montana State University in Bozeman, were looking
for other clues in the rocks to interpret the environment at the time
these eggs were laid. Tall and energetic, Jim strolled all over Auca
Mahuevo pointing at numerous important geological features that
would clarify our view of the environment and preservational aspects
of the nesting site. Specifically, David and Jim had been examining
two extensive, whitish rock layers a few feet above egg layer 4. David,
a man of few words, is the kind of researcher who gathers a lot of data
before reaching a conclusion.
On March 16, David and Jim told us that those whitish horizons
were enormous layers of dinosaur tracks, surfaces that had been
stepped on by thousands of sauropod dinosaurs. To confirm this
interpretation, they wanted to remove the overburden of clay across
a large area and examine the whitish horizons on a fully exposed sur-
face. On March 18, several members of our crew worked down
through the overburden on a long, low hill until they reached the
whitish rock layer. We saw, to our amazement, that the horizon was
made of distinct bowl-shaped structures containing a white min-
eral. These white bowls were distinct from the reddish clay that sur-
rounded them. With this piece of evidence, the interpretation of our
geological team was fully confirmed. The whitish horizons repre-
sented wet surfaces on which the dinosaurs had walked. The large,
bowl-shaped depressions formed under the weight of their feet
remained exposed for years, accumulating a shallow film of water dur-
ing the wet season. With the evaporation of this water, a variety of pre-
cipitates added a layer of white minerals to the bottoms of the bowls.
Because these surfaces would have been exposed for many decades,
the deep dinosaur tracks would have accumulated the one-to-two-
inch-thick deposits of mineral precipitates that we were observing. The
lateral extension of these track horizons helped us connect up some
of the discontinuous exposures of egg layer 4 and gave us a better sense
