David,

That's quite a bit of erosion. Imagine standing there at the bluff with your
finger on the contact between the Cretaceous and Pliocene sediments. The
Cretaceous shells are more than 65 million years old, the Pliocene shells
are only 3 to 10 million years old, more or less. That's a staggering gap --
not a page or two of the earth's history, but a whole long chapter ripped
out of the book, with one word left: a single Cubitostrea shell dating to
about 45 million years ago.

Erle Kauffman told an even more spectacular story from nodules lifted off
the surface of the submarine Blake Plateau east of Georgia (USA). The
nodules had formed as the sediment filling ammonite shells had hardened into
concretions. They were exhumed and left lying on the surface as the
surrounding soft sediment was winnowed away. Animals bored into the nodules
and encrusted their surfaces. The nodules were buried again; they grew into
the surrounding sediment, and the sediment inside the borings also cemented
and hardened. They were exhumed again, bored again, encrusted again... six
episodes of submarine exposure in all, each one attested by a different set
of microfossils from the fills of borings. Six, that is, if you include
their current exposure on the Blake Plateau. The history of this part of the
Earth is highly condensed, and we call the thin veneer of sediment that
includes the nodules a "condensed section." Condensed sections can carry a
lot of history, but it's like reading a page from this chapter and that.

It's a pity that so much of the record has been lost, but... if the Earth
kept the equivalent of all its old National Geographic magazines, its garage
would be full now, wouldn't it?

Andrew K. Rindsberg
Geological Survey of Alabama