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Re: Farrington's Circle

At 03:56 PM 6/2/98 -0500, you wrote:
>According to a site I've found, there is a significant anomaly in the
>distribution of iron meteorites in the southeast United States. Can
>someone back this up? Beyond this single page, there is no mention of
>Farrington's Circle anywhere on the web.
>
>http://www.research.umbc.edu/~frizzell/farrington.html
>
>
>
Listoids:

Here is the actual distribution of meteorites (falls + finds) in the
US.

Http://www.uark.edu/studorg/metsoc/temp/usmets.gif
  (legend: red=irons, green=stony-irons, blue=stones)

Oliver Farrington was a Curator at the Field Museum in Chicago in the
early 20th century.  I haven't looked up his papers, but I do have a
copy of his 1915 book "Meteorites" here in my office.  He was interested
in the fact that meteorites tended to be found near mountain ranges
(the Himalayas, the Alps, the Appalachians), and in particular that
iron meteorites seemed to be concentrated near mountain ranges as in
the southeastern US.    He says,

  "Whether such regions exert a greater gravitational force or 
   whether they present an actual physical obstacle
   to the passage of a meteorite is uncertain, but it is probable 
   that if either agency is operative it is the obstructive one."  

He goes on to try to explain the excess of irons in the western hemisphere
compared to the eastern hemisphere,

  "Berwerth has sought to account for the excess of irons in the
   New World by the suggestion that the dry air of the desert areas
   which abound in this hemisphere has preserved meteorites fallen
   in long distant periods while those of a similar age in the other
   hemisphere have been exposed to a moist climate and have for the
   most part decomposed.   It is true that many of the iron meteorites 
   known from the western hemisphere occur upon the Mexican and 
   Chilean deserts, but quite as many come from the southern
   Appalachians, where a comparatively moist climate prevails.  There
   are also numerous desert areas in the Old World perhaps as
   fully explored as those of the New, so that on the whole the
   above explanation seems inadequate."

And that's all he has to say on this subject.

We now can say with confidence that both Farrington's gravitational
and obstructional explanations for meteorite concentrations are
wrong. To a very good approximation, all meteorites fall with equal
frequency everywhere on Earth.  

He was certainly on to something in that second quote about deserts.
For a complete explantion of the distribution map of meteorites
we need to look at three major factors (that I can think of).
   1) Climate.  It is certainly true that dry climates preserve
meteorites better than moist ones.  Moreover, moist climates
product abundant vegetation and deep soils, both of which obscure
meteorites.
   2) Geology/hydrology.  These can influence our ability to find
meteorites in several ways.  Certainly, an abundance of terrestrial
stones and cobbles makes it much harder to find meteorites visually.
Such areas may product more irons than stones, since the irons don't
look much like rocks. The color of the rocks can also help or hinder 
locating meteorites.  It's possible that certain soils have chemistry 
that is more or less destructive to certain types of meteorites.  
Sedimentation rate is also a key factor in determining whether meteorites 
are found.  Areas with few loose rocks and low (or negative) sedimentation
rates are the best places to find meteorites.
   3) Human culture.  Clearly, people play a great role in determining
where meteorites are and will be found.  Areas that have had long and
dense occupation by humans tend to have fewer new meteorites found.
Presumably, farmers found most of the irons on their land long ago, 
perhaps putting them to some good use, either as tools or otherwise
(e.g, as religious objects in burials).  I believe this is why there
are so few irons in places like Europe and New England.  It also
takes lots of people living in an area to recover falls efficiently.
This is why Europe, Japan, and India have so many falls.  Meteorite
collectors and scientists also greatly affect the distribution of
finds.  Harvey Nininger was almost singlehandedly responsible for the great
"concentration" of meteorites found on the American prairies, as he
mobilized farmers to look for unusual rocks (taking advantage of the
geological factor, above, that these areas lack many natural rocks
and stones, which is why there are so many chondrites there).

So I think that you can look to these factors to explain the apparent
concentration of irons in the southeastern US.  The area is heavily
agricultural, even in mountainous parts of the reqion.  This can
lead to many finds.  It has a reasonably large population, but is
not so long occupied that most of the irons will be gone.  The
Appalachians are quite rocky, favoring irons over stones.  The
coastal plain, on the other hand, is a lousy place to find any
meteorites (having high sedimentation, deep soils, heavy precipitation,
swamps, etc.)  Put all of this together, with the great abundance of
stones in the midcontinent (as discussed above), and I think you
have the explanation for "Farrington's Circle."

For the most part.

-jeff