When leaving the New York
State Thruway on Exit 21 B you can see on the east side of route 21 W a
large
and almost vertical rock surface covered with these structures. Locals
refer to
it as “dinosaur skin”. It is also a favorite stop on excursions of
students and
professional geologists. They know that these rocks are part of the
Ordovician
Normanskill Formation and that they formed not only long before the age
of
dinosaurs (470 million years ago) but also on a deepsea bottom, way
beyond the
reach of any air-breathing diver. Such rock sequences, called “flysch”
after an
occurrence in the Swiss Alps, are always folded. This is because
deepsea
sediments become normally subducted and re-melted with the oceanic
crust on
which they were deposited. They escaped this fate only where the
lighter and
softer sediment cover was split from its basaltic base and thrust onto
the continents
as folded mountain belts. This led to the paradox that today deepsea
deposits
often crop out thousands of meters above sea level and hardly ever in
their
original horizontal position. The Normanskill rocks became upthrusted
when an
island arc like modern
In
the depth of the Ordovician ocean that became closed in the Taconic
orogeny,
sedimentation went on as in all deep ocean basins: long periods, in
which only
fine ooze, volcanic ash, and the rain of dead planktonic organisms
accumulated
in millimeters per century. In the marginal areas, however, a huge
avalanche of
suspended sediment came down the continental slope every once in a
geologic
while. Such a turbidity
current first
eroded the upper layers of ooze and then redeposited it in a graded
fashion,
the imported sand first and the suspended mud on top.
The
“dinosaur-skin” surface is the sole of such a turbidite. Guide books
refer to
it as flute casts,
sedimentary
structures that originate by a kind of erosion taking place
just before
the first layer of sand comes to rest from a turbid flow. In this
moment, small
eddies form and act, together with the sand grains, like minute glacier
mills.
Accordingly, each flute starts with a globular excavation, whose
drawn-out tail
on the downcurrent side marks the migration and eventual dissipation of
the eddy.
Flute structures were never observed on modern sea floors, because they
become
immediately buried under the sand that scoured them.
The
present structures differ from typical flute casts. Instead of
resembling
radishes, they have a brain-like appearance. Also unusual is their
arrangement
in stretches perpendicular to current direction. This kind of
sedimentary
structures could be called “mop
casts”,
because the individual depressions end in a rather sharp front on the
downcurrent side and
thereby resemble the threads
of a household mop. The difference is important for reconstructing of
the
ancient current direction: the steep sides point up-current in flute
casts, but
down-current in a mop cast, as shown by crossbedding in the overlying
sand.
For
these structures to form, the mud surface must first be modeled into
erosional
ripples (normal ripples form by accumulation and tractional transport
of sand).
On such a rippled surface the sand comes to rest first on the lee
slopes.
Driven by gravity and currents, it will then slide downhill and stop
near the
base of the slope. At the same time, small regions of the heavier sand
sink
deeper into the mud, like in another group of sedimentary structures
called load casts.
So two processes, creeping and
loading, are combined in the mop casts. The associated drag marks are parallel
to current direction;
they were carved by large objects or rock fragments carried by the
turbidity
current, but only after the first sand had settled.
These
details are
important for the geologist reconstructing the slopes in ancients
basins.
In our fantasy, however, this
structure
remains dinosaur skin and another example for the creative powers of
natural
processes.