Re: stratigraphy in my backyard

Dear Jawod:

<[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> Again, an amateur request for help.
>
> Can someone steer me in the right direction to determine the age of a
layer
> of limestone in a creek-cut hillside in my backyard?
>
> Being in Cincinnati, I know I am near (but not too near) the Cincinnati
Arch
> of Ordovician layers.

Your thinking is obviously systematic. I think you can do the required
sleuthing on
your own if that is a prospect which interests you. I would use a search
engine to
check for geological maps on line along with information on the paleontology
of the
area.
Derived from your own writing, some terms you may want to mix in the search
would
be +geology  +"geological map" +paleontology +fossils +Cincinnati
+"Cincinnati Arch" +"*** Ohio,
etcetera. The asterisks would equal the sectional compass area of Ohio you
are in.

> I don't know the fossils very well.

Shimer and Shrock are famous for their work "Index Fossils of North
America".
See your search engine, library or Amazon at
www.amazon.com/exec/obidos/tg/detail/-/026219001X?v=glance
for more details. An index fossil is generally a species which existed for
only a
small interval of geological time but occurs across an ample geographical
area. Index
fossils are accordingly great indicators of the age of the host rock,
assuming they have
not been reworked (redeposited) into sediments of a later time period.

>This particular
> formation is about 15-20 cm thick and lies between much thicker layers of
> soft clayey shale of medium gray with very few fossils.  This is in a
> drainage area to the Little Miami River maybe ten miles north of its mouth
> on the Ohio.  I am thinking it is much less old than 450 MYA...but how
much?

The Ordovician Richmond Formation is plentiful in your area and sounds very
close to
what you describe. You will find a good bit on it online.

> I am guessing that for most of the period while this rock was laid down
was
> turbid and muddy: the clayey shale.  There was a period of clear water
with
> a prolific marine environment: the high density of shell debris and other
> fossils in the limestone.  Am I close?

High energy deposition or turbidity is usually associated with a lot of
broken fossils.
Is that what you see? Taphonomy is the study of conditions and processes
relating to
fossilization---another search word for your pursuit.


> The coolest thing, though, is that the under-surface of a large rock I
> pulled out of its position in the cut has these great casts of indented
> trails made by some crawling thing ...how long ago?  So, I am thinking
that
> these trails occurred at the transition from a muddy swamp or something to
a
> clear water environment at that time.

I'd have to see the specimen to join in your verdict. Carefully examine both
sides
of any broken rock face to make sure there are no hard fossil structures in
one making imprints in another. Fossil impressions are called trace fossils
or ichnofossils. There are a number of different trilobite impressions made
by digging, walking, nesting, etcetera and there are known occurences of
these in similar sediments from your area. Try looking at Google or Dogpile
for images of cruziana(trilobite trackways) and rusophycus (trilobite
resting places) to start. Here is a starter page on ichnology:
http://www.geo.arizona.edu/geo3xx/geo308_fall2002/backup/cha3.html

> What is the best approach to solve this puzzle?  I suppose I could lug the
> rock down to the Univ of Cincinnati Geology Department...what would they
ask
> me regarding the original location? anything else I should tell them?
>
> Thanks very much for understanding my enthusiasm and for looking past the
> lack of training!

I think you will have little trouble effectively pursuing this if you take a
little time. The
web is replete with sources for you. The U of C is heavily involved with
area paleontology. It
is the consensus choice in your area for institutional resources. There is
also an amateur fossil
club with a good reputation in the vicinity called the "Dry Dredgers". A
number of knowledgeable
folk belong who would be of aid to your quest.

Jo made other fine suggestions for you to consider.

Good luck and regards,

Edward Hennessey

nohalozzyzxnohalo.noyahoo.com minus no,no,no.