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Austin Wash

Cannot identify age of fossil bed in Virginia, USA

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Austin Wash

This might prove very easy for more advanced fossil collectors to answer. In 2004, the floodwaters from Hurricane Gaston swept away a large amount of soil and clay from an existing stream near the backyard of our suburban house near Mechanicsville, Virginia, exposing a clay bed littered with numerous fossilsIMG_1999.JPG. The turritella you see in the picture occurs the most frequently of all our finds, and the small clam fossils are a close second. We've recently started to find more of the kind of scallop fossil in the image, which we guessed was a chesapecten jeffersonius, Virginia's state fossil. I found one moonsnail fossil in the same clay, but it's the only fossil of that kind that we've found.

Anyway, I'm not much of a geologist, so I haven't been able to precisely date these, or identify them with a specific epoch. I have what I think is a reasonable guess, but I'd like to get a specific date on just how many years worth of soil Gaston scrubbed away from our backyard. Thanks!

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Doctor Mud

Hi Austin Wash,

 

Welcome to the forum,

 

Sorry I don't know the geology of your area. Someone may just know off the top of their head. Sounds like you are well on the way to figuring out the age using index fossils which can be useful for pinning down ages. A geological map would also be useful and often they are available online. The fossils look like what we get in the Pliocene - Pleistocene sequences in New Zealand though.

 

regarding the soil loss. Even if the soil was on top of this unit you would have to be careful of saying this rock is (for example) 1 million years old, therefore we lost 1 million years of soil. Rock and soil layers aren't continuously deposited through time. You can have periods of no deposition and erosion which result in gaps in the geological record known as unconformities. 

 

oh by the way - jealous that you have an outcrop in your back yard!

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Plax

That pecten is early Pliocene I think.. You can google the state geological map for more accurate information.

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Fossildude19

Generalized geologic map of Virginia. 

 

51d40c1363edf3543966bad2750e42ce.jpg

 

 

 

EDIT: According to this website, Hanover County has the Aquia Formation exposed. 

 

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Austin Wash

IMG_2005.JPGThanks for the input, everyone! Here's another image, plus a random observation that I found interesting:

 

Everything on the left side of the pen in the image are things we've found in the random outcrop in our backyard. At the top-left of the left side is the only moonsnail I've found in the area, and at the top-right is the largest scallop we've found. At the bottom-right is one I found just today, after a pretty large amount of rain. The pieces to the right of the pen, however, were all found at a different location several miles away, during a field-trip I went on with the Geology department at Randolph-Macon College. Note the ecphora, which is a pretty good indicator of the age, as well as the scallops and barnacles. The two pieces at the bottom are two halves of the same scallop, by the way.

 

What I find interesting and weird, however, is that all of the scallops we've found at our location seem to have about 16 individual ridges, whereas those to the right of the pen have about 10 each, and have less narrow ridges than those we've found. Part of why this is confusing is that those with about 10 ridges were identified as the jeffersonius by the professor leading the trip, and it makes sense to see them alongside the ecphora. I can tell you the site from which we recovered the ones on the right was much more diverse than ours, as it also included numerous different gastropods (interestingly, however, very few of these were the sort of turritella, of which we can find at least one per square inch of our location).

 

Anyway, I was wondering if anyone could guess why it is that the scallops we find have a pretty consistent 16 ridges per individual piece. The explanation may be pretty simple, but I'm not sure how simple/not simple the difference is. I am by no means an expert on scallops, or any marine life for that matter.

 

I've also wondered about what sort of habitat our outcrop was likely to have been. I've noted before that I've only found one moonsnail in the area, which corresponds pretty well with the lack of the snail's distinctive bore-holes found in bivalves where the snails are more frequent (these bore-holes and the moonsnails themselves were MUCH more frequent in the area from which the pieces on the right of the table were taken). The turritellas absolutely litter our outcrop, however, and other sorts of mollusks are pretty infrequent finds compared to the turritellas and scallops. Also, I've only ever uncovered two barnacles from our location of the sort that can be seen on the scallops on the right of the table. If anyone knows what could account for the differences, I'd be interested to know.

Thanks again!

 

 

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Fossildude19

My guess would be a different species of Chesapecten - something like Chesapecten madisonius?

See this thread for some more info. 


Also, maybe @Al Dente or  @MikeR or @MarcoSr could tell you something more.

 

Regards,

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Fossildude19

Mike, 

Thanks for weighing in on this. 
I was out of my depth, and had hoped one of you more experienced members more familiar with the area and available formations would help us out with this. 

Once again, sir, your knowledge is appreciated! :)

Regards, 

Tim

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Fossildude19
41 minutes ago, MikeR said:

 I have a description between the two units in my blog here Rushmere.  Ignore the picture quality.  The forum server switch a few years ago does not allow me to edit older entries.

 

Mike

 

 

Mike - feel free to message me with any changes you want made - I can switch out any photos you would like me to. ;) 

Best regards,

 

Tim

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