Geoscience Reference
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that, near land, may be more than a metre thick. Geologists call these
storm layers, or tempestites—a Shakespearean resonance quite in
keeping with the drama involved.
Farther out to sea, in deeper water, small fair-weather waves do not
reach down to the sea floor. This is the realm of the storm waves that,
in rough weather, create a different and rather puzzling structure. Not
quite a set of horizontal layers, and not quite the steeply inclined lam-
inae of fossilized dunes—rather, packets of layers, usually a few tens
of centimetres thick, which seem to be more subtly wavy, to pinch
and swell, often in rhythmic sets. When the top of such stratal sur-
faces are exposed, with a bird's-eye view they look a little like the top
of an enormously magnified golf ball, showing rhythmically arranged
gentle hummocks a metre or so across and a few centimetres high.
Nothing spectacular—and easy to miss if you didn't have your eyes
wide open. Peer at the bottom layers of these subtly dimpled units,
though, and they are not as gentle as they look. They are often full of
smashed and broken seashells.
Hummocky cross-stratification, this type of layering was termed,
and it turned out to be so common as to be given its own acronym:
HCS (geologists, alas, have a predilection for this kind of abbreviation).
What formed it? This type of layering is now most convincingly inter-
preted as the results of great offshore storms. HCS should also be a
planetary constant. It should be high on the checklist of clues to be
sought by the Mars rovers as they trundle across the strata of the red
planet. If they do ever find HCS on their travels, then that acronym will
become headline news worldwide: a sure-fire indication that this now
freeze-dried planet had, once upon a time, deep and stormy seas.
Living Oceans
The oceans of an icehouse Earth circulate energetically. On a world
with abundant polar ice there is never a shortage of freezing winds to
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