Geoscience Reference
In-Depth Information
tive, and similar layers can be found in ancient sequences of rocks,
testifying to their origin during volcanic eruptions. When petrified,
these layers of ancient volcanic ash are often called tuffs. We had
hoped to find these tuffs in the sequence at Auca Mahuevo in order
to provide environmental clues and to establish the age of the rocks
through radioactive decay analyses.
Similarly, rivers and streams can be seen to deposit layers of
gravel, sand, silt, and mud as they make their journey from steep
mountain canyons across gently sloping floodplains. The kind of
sediment that a stream can carry depends on the velocity and turbu-
lence of its currents. Swift, turbulent currents flowing down steep
slopes can transport boulders, as well as smaller particles of sand,
silt, and clay, but less turbulent currents cannot carry such large sed-
imentary debris. Consequently, coarse, heavy sediment is often
deposited by swift rivers and streams near the mouth of a steep
mountain canyon, where the slope of the stream flattens out. These
boulders, pebbles, and coarse sand grains are dumped into the val-
leys adjacent to the mountains as alluvial fans, while much of the
fine sand and mud is carried on downstream. Immense alluvial fans
can be seen today at the mouths of canyons leading into Death Val-
ley in California. As mentioned earlier, the gravel-rich layers behind
Dona Dora's
puesto
represent the kind of coarse gravel that was
deposited on alluvial fans adjacent to some ancient hills or moun-
tains on the Cretaceous Patagonian landscape when titanosaurs
roamed across southern South America.
On more gently sloping plains farther away from the highlands,
often on the coastal plain, slower, less turbulent rivers and streams
deposit lighter sediment in stream channels, flood basins, and lakes.
The coarser sand often forms sandbars in the stream channels. Finer
silt and clay is often deposited outside the banks of the channels dur-
ing floods, when the rivers and streams overflow their banks and the
slower currents carry the silt and mud far away from the main chan-
nels before it settles out. Such deposits of sandbars in channels and
mud on the floodplain away from the channels have been observed
throughout many modern river systems, and similar layers of sand and
mud can be found in many ancient rock sequences. These are prime
places to look for fossils of dinosaurs, as well as other animals and
plants that inhabited the ancient floodplains, because their bones and
