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had noticed that, unlike other clutches, this one had been laid on a
sandy substrate left by an ancient river that had dried up before the
dinosaurs had laid their eggs. He had also noticed that the eggs
appeared to have been laid in a somewhat rounded hole in the sand,
which was covered by greenish red clay. Alberto had made another
exceptional find.
When we got to Alberto's site, we immediately knew that we were
looking at the first-known well-preserved sauropod nest. More than
twenty-five eggs had been laid in a depression surrounded by a tall,
sandy rim. Close examination of this depression revealed that the
cross-bedded sands initially deposited by the stream had been dis-
rupted, and that the rim of the nest was formed by massive, struc-
tureless sand, all of which indicated that the depression had been dug
by the female dinosaur who had laid the eggs. The clay covering the
eggs was vivid testimony of the flood that had later inundated the area.
It was also significant that this nest was not located in egg layer 3, the
bed of eggs that we had explored most extensively, but rather in a
stratigraphically higher layer. After a couple of days of hiking, the geo-
logical team established that the nest was in the same rock layer as egg
layer 4. This confirmed our suspicions that the same kind of natural
catastrophe that had buried the nesting colony of egg layer 3 was
responsible for the demise of the nests in egg layer 4.
During the following days, three other examples of nest struc-
tures were found across six hundred feet of the sandy bed of this aban-
doned river channel. All of these nests exhibited the same layout, with
eggs contained in round or more irregular bowls, about three to four
feet across, surrounded by elevated rims. This evidence convinced us
that all the other clutches we had found had originally been laid in
depressions excavated by the females. We had not recognized this
before because, in all other instances, the females had chosen the
muddy substrate of the floodplain to lay their eggs on. This had pre-
vented us from observing any differences between the clay in which
the eggs had been laid and the clay in which the eggs had become
buried in the flood. Fortunately for us, a handful of females used the
sandy bed of an abandoned river channel to lay their eggs. Because
these nests were dug in sand and covered by flood-generated clay, their
original structure became detectable to the geologic eye. This find not
only gave us our first glimpse of the structure of a sauropod nest but
