Geoscience Reference
In-Depth Information
ancient sand dunes who had accompanied us on expeditions to the
dinosaur-rich deposits of the Gobi Desert in Mongolia. There, his
observations were critical in helping us understand how the beautifully
preserved fossils had come to be killed and preserved by massive,
water-soaked sand avalanches that had slid down the dune faces dur-
ing occasionally heavy rainstorms. His extensive knowledge about how
different kinds of rocks form would also prove invaluable to our stud-
ies of the rocks that entomb the eggs and embryos at Auca Mahuevo.
Dave immediately thought that the slickensides might be related
to a geologic process called vertisols, which operates in some soils. He
explained that, in clay-rich soils, alternating cycles of wetting and dry-
ing can lead to expansion and contraction of the material in the soil.
The expansion occurs when it rains and the clay absorbs the moisture,
which causes blocks of the soil to move upward, creating the slippage
surfaces or slickensides that we had observed. This "vertical" move-
ment occurs because the direction of least resistance is upward, since
rock or dirt is present on the sides and below the soil, but only the
atmosphere or a thin layer of soil overlies the expanding clay. The ver-
tisols would probably have formed within decades or a couple of
centuries after the floods deposited the mudstone.
The vertisols appeared to have some other important implica-
tions. First, they offered additional evidence that the area occasion-
ally received substantial amounts of rain, alternating with substantial
periods of dryness. Since we suspected that floods had killed the
embryos, the vertisols would help establish that rains potentially
generating floods did occur. Also, vertisols could easily have led to the
development of a hummocky surface on the floodplain, with small
bumps and depressions littering the landscape, and such depres-
sions might have made good places for the sauropods to lay their
eggs. Finally, the vertisols could explain why many of the eggs were
somewhat squashed and some of the egg clusters appeared to be
mixed up with one another. Movement along the slippage surfaces
could have created these effects by crushing the eggs trapped in the
moving blocks and displacing others.
Frankie's mapping in the quarry gave us a good idea about how
individual eggs were distributed in clusters, but the quarry was too
small to document how the clusters were distributed across the land
surface. To determine how the clusters were distributed, Luis,
