Geoscience Reference
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ing out of the same layer of mudstone that had contained the eggs on
the flats and at the embryo quarry, but we had to check out Brooks's
new site to be sure. As we walked east about a half mile from the road
to the ridge, it soon became clear that the eggs Brooks had found
were at a much higher elevation than the eggs on the flats. We knew
that more hiking would be required to determine where the new
layer fit in the sequence of layers, especially in relation to the layer
that contained the eggs on the flats, but we would have to wait until
our next expedition to conduct that detailed exercise.
One of our primary geological goals when we returned to Auca
Mahuevo in March 1999 was to make sure that Brooks's eggs came
from a new layer. To do that, Lowell, Julia, and Alberto traced the
layer of mudstone that contained the eggs on the flats to the east
along the base of the ridge where Brooks had found the new eggs. The
egg-bearing layer on the flats ran right along the bottom of the ridge,
whereas Brooks's egg site was situated near the top. More than sev-
enty-five feet of mudstone and sandstone layers separated the two
layers containing eggs. Consequently, we knew that the sauropods
had laid the eggs preserved on the flats long before they had laid the
eggs at Brooks's site.
At present, we cannot tell how long the eggs on the flats were laid
before the eggs at Brooks's site. It's not likely that that much sedi-
ment could have been deposited within a few years or decades, but
whether that seventy-five feet of rock took centuries, a few millennia,
a few tens of thousands of years, or even a hundred thousand years to
accumulate is unknown. Because we have not found any ancient
layers of volcanic ash at the site to date through radioactive means,
we just cannot be sure. We could not use the magnetic information
contained in the rocks to make such estimates because all the rocks
we have sampled thus far at Auca Mahuevo were deposited during an
interval when the earth's magnetic field was reversed. Nonetheless,
because of the law of superposition, we can be sure that the eggs con-
tained in the mudstone layer at the flats and the embryo quarry rep-
resent a distinctly earlier page in the geologic history of Auca
Mahuevo than the page represented by the layer containing Brooks's
eggs-
It might seem a bit strange for a photographer to discover an
important new locality laden with fossil dinosaur eggs. That was
