A Basic question of Fossil Deposition in a Stream

[JamieLynn]

So this is probably a very basic question, and I probably should know this since my Dad is a river geographer (I know what a thalweg is and can locate it, so at least there's that!) but since it's specifically relating to fossils...I figured y'alls collective knowledge might help me out! 

 

I have been finding tiny little Shark Ptychodus teeth, the lateral files, all about 1/4 inch or smaller, at this one particular creek spot. But I have not found any of the regular sized teeth., which typically range from 1/2 to 1 inch or more.

 

So should I be looking further UPSTREAM or DOWNSTREAM?? If they are coming from upstream, would the larger heavier teeth drop first or would they get tumbled further downstream? Just trying to figure out my best chance of finding some big uns! Thanks for any insight! 

[hadrosauridae]

Larger items always drop out and get trapped first.  I would start looking upstream.

[ThePhysicist]

Find big rocks, find big teeth.

 

I would say the question is ill-posed. Upstream or downstream is largely irrelevant for your purposes especially if you don't know where the fossils are sourced from, though by all means look everywhere you have time for.

 

You are however right to think about weight. Rivers sort their load according to the water speed. High speed flow can carry heavier particles than slower, low energy flow. Deposition occurs when the water speed is no longer sufficient to transport the particle. This means that particles of roughly the same weight (~size, assuming similar density) will be deposited in the same area. You will want to look in the deposits with a grain size comparable to the tooth size you're looking for - since water of course doesn't care whether it's a rock or tooth. This could be upstream or downstream or where you've already been. Hope this helps.

[DPS Ammonite]

Sometimes you need to look at different parts of a stream. Heavier fossils may be found in localities where placer gold is found. Also, larger fossils may be found in the  deeper part of main channels. Smaller fossils found in areas away from main channel where the current is slower.

 

https://earthsci.org/mineral/mindep/auplace/auplace.html#:~:text=Gold will collect at the,can be places of deposition.

[LSCHNELLE]

Let me just tag into the conversation a perspective about the larger half inch to 1+ inch teeth. My experience is Ptychodus frequency is about 1:30 (>15 to 25 mm) and  1:140 (>25 mm) for tooth width. Also note, most Ptychodus teeth in a complete dentition are of the smaller variety. Possibly as much as 60 to 70%. So you're more likely to find the smaller ones. 

 

As to your question about depositional systems, I think the other TFF members have hit on the main issues. That is, if you have one associated Ptychodus dentition being eroded away upstream, then maybe you could be looking upstream for larger (and more) teeth. But, if it has already been randomly washed around by erosional forces in a prehistoric ocean or stream, then there are more random and multi-directional sorting mechanisms at work. So, there's no way to know for sure which direction to look for the next big tooth.

Edited August 23 by LSCHNELLE