Hi Andrew G,

>>But certainly, in porous
>>shell beds, the weathering zone will be deepened by long-term acid
>>rain. The fresh part of the shell bed, on the average, will be farther
>>underground than before. As collectors, how should we respond to this
>>trend?

>Get out and collect all the fossils...?

;-) The obvious response of the inveterate collector -- but an irresponsible
one!

We don't even know that there's a significant problem. It may be extreme in
Holland and negligible in New Zealand -- but people have mapped acid
precipitation, so we don't have to do that over again. What we need is some
way to measure weathering rates in shell beds -- and a better understanding
of what happens chemically as shells weather, for that matter. One might
choose a few wide-ranging genera (perhaps an ostreid, venerid, and
turritellid) to act as a standard, since shells of different families most
certainly weather at different rates, depending not only on mineralogy, but
also on the arrangement of crystals within the shell. Conids, for example,
turn to mush while most other shells remain intact in the Alabama Coastal
Plain, while oysters last for years sitting on top of the ground.

One would certainly want to compare new and old photos of outcrops, but
there are so many variables involved. Outcrops undisturbed by collection
tend to develop a weathering rind that can slow down weathering behind it;
continually disturbed outcrops look a lot fresher. The first person to visit
Little Stave Creek, for example, probably saw bluffs with a reddish-brown
weathering rind containing somewhat weathered shells, perhaps 6 to 12 inches
(15 to 30 cm) thick. At present, the most-collected beds are deeply gouged
and the exposed material is dark gray and fresh. (Before you ask, this
famous Eocene outcrop is still closed to visitors, because of abuse by
shark-tooth collectors.) We would therefore have to careful while comparing
old and new photos.

It WOULD be worthwhile, in this digital age, for people to photograph
outcrops more often, and to annotate directly on the photos where they
collected. Be sure to include something to indicate scale -- a metric scale
is best, but a yardstick or anything of standard size, such as a geologic
pick or even a spouse of known height, will do.

And has any collector ever measured the pH of groundwater going into and
coming out of a shell bed? Seems like a great project for a high school or
college student, since it would be easy to do, at least in the rainiest
season (that would be December to April in the southeastern U.S., hint
hint). It would be publishable, too.

Cheers,
Andrew

Andrew K. Rindsberg
Geological Survey of Alabama