When leaving the New York
State Thruway on Exit 21 B you can see on the east side of route 21 W a
and almost vertical rock surface covered with these structures. Locals
it as “dinosaur skin”. It is also a favorite stop on excursions of
professional geologists. They know that these rocks are part of the
Normanskill Formation and that they formed not only long before the age
dinosaurs (470 million years ago) but also on a deepsea bottom, way
reach of any air-breathing diver. Such rock sequences, called “flysch”
occurrence in the Swiss Alps, are always folded. This is because
sediments become normally subducted and re-melted with the oceanic
which they were deposited. They escaped this fate only where the
softer sediment cover was split from its basaltic base and thrust onto
as folded mountain belts. This led to the paradox that today deepsea
often crop out thousands of meters above sea level and hardly ever in
original horizontal position. The Normanskill rocks became upthrusted
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
remains dinosaur skin and another example for the creative powers of