Screening Streams

[Carl]

I've been wondering lately about the distribution of streams in which fossils are screened from the gravels and I only know of ones along the eastern coast of North America. Can someone fill me in on others around the world? I'm not thinking about streams with rocks that contain fossils within them as much as streams in which people screen out whole matrix-free fossils. Thanks!

[Damage]

I have access to a few streams but am not sure if it's even worth looking. There seems to be plenty of pebble and gravel in the streams but I don't know if I'd find anything or not. Any tips for where to look in streams?

[FossilDAWG]

An important factor on the coastal plain of the Eastern US, and also the gulf coastal plain (Alabama, Mississippi, Texas in places) is that the "bedrock" is mostly unconsolidated, so certain hard fossils such as shark teeth can easily erode out and accumulate in the stream bed. Note also that the fossils people screen for are pretty tough, such as teeth, rather than shells, so you'd need unconsolidated deposits with significant concentrations of teeth. I know of no such place in Canada, for example, because the bedrock is hard rock and fossils rarely erode out intact, and/or the stream bed is full of loose rock that bashes up any loose fossils. There are places where nodules/concretions accumulate; in that case the fossils are protected, and people don't screen gravel to collect the nodules but rather dig or just pick them up.

Don

[Auspex]

Any Texans here screening in the North Sulfur River or Post Oak Creek?

[Joe D.]

The streams pretty much have to cut through a loam or marl/clay strata that holds fossils to be capable of depositing intact fossils. Same goes for the bed of the water course. Geologic maps can only help if they get very precise in identifying where such layers may appear. There are plenty of places that held old swamps and peaty bogs that would hold great fossils if they existed in the proper geologic time period and haven't been converted to solid rock.

A smart Geologist should be able to head you in the right direction. Water courses are my best friend when it comes to unearthing great rocks, minerals and fossils. When the water courses have to erode through solid rock they don't leave much in the way of intact fossils. I always told newbie rock collectors to pan a stream to find out what it might hold, that goes for fossils also. Streams that run by old closed sights can be treasure chests just waiting to be opened.

Joe D.

[Carl]

Sorry, I'm not really looking for an explanation of screenable stream dynamics or how to find them - I'm just curious about geographic range and commonality. I forgot about things like the Sulphur River in TX - thanks Auspex!

[Plax]

Hey Carl,

When we moved to NC in '92 I didn't see anyone screening streams. Of course there are hundreds(?) of collectors doing it now. I think the idea has to catch on in each region and the pickings have to be more rewarding to promote screening. I think most sites are sight picked for a few years first before screening becomes obviously profitable. The collectors find fewer teeth on the surface due to collecting pressure and this stimulates screening technology. This applies to placer deposits. Mother lodes along stream banks must be screened of course. Geology has already been mentioned as a prerequisite. My comment hasn't answered your question regarding where else fossils are screened from stream gravels. Would also be curious as most of the forum postings on the subject come from the east/gulf coast as you've mentioned.

Don

[Carl]

I'm really interested in learning about screening streams off the Atlantic and Gulf Coastal Plains.

[BobWill]

There's a lot of screening at Post Oak Creek in Sherman Texas, but I don't see much it at North Sulphur River. That's not to say other methods aren't used. Some people just sit down on a gravel bar at Post Oak and dig in one spot. Others walk or crawl along and pick teeth up from the surface. Still others take home buckets of gravel to go through with magnification for the micros. I've also seen a technique that requires a little more vigor. They choose a gravel bar with a slight slope toward the creek and use a shovel to stir up the surface. Then they throw buckets of water on the area which runs back into the creek, washing away the sand and revealing many teeth you would never see otherwise. I haven't collected on the coast here but I've talked to people who screen on the Texas gulf coast so it must be at least somewhat productive.

[Pagurus]

I can't help with stream locations but this thread reminded me of a question I've had awhile about streams. Are larger fossils generally found among larger pebbles and smaller fossils found among the smaller pebbles and gravel? I've found a few larger shark teeth among larger pebbles but I don't have enough experience to come to any conclusions myself. I'm heading down to New Jersey later this week and looking forward to sharpening my limited skills.

[BobWill]

I can't help with stream locations but this thread reminded me of a question I've had awhile about streams. Are larger fossils generally found among larger pebbles and smaller fossils found among the smaller pebbles and gravel? I've found a few larger shark teeth among larger pebbles but I don't have enough experience to come to any conclusions myself. I'm heading down to New Jersey later this week and looking forward to sharpening my limited skills.

This is generally right because rocks (and fossils) fall out of moving water when it slows enough to no longer support their weight, so water moving at a certain speed drops material of a uniform weight out in the same place. This happens as a stream bends around a curve in it's course because the water wants to all move at the same speed but the water inside a curve has less distance to travel than water on the outside of the curve. The slower moving water inside a bend tends to drop material being carried along just as the faster moving water on the outside of the bend is often removing rocks and dirt from the bank to carry downstream.

[Carl]

I agree with BobWill but I have learned that there is another factor here to consider. Pebbles and shark teeth have very different hydrodynamic profiles and overlain on the size sorting dynamic is the different behaviors of the shapes of the objects. I'm not sure how this helps but it is something else to consider.

Hey! Anyone out there know ANY screening streams outside of the US????

[Pagurus]

Thanks, BobWill for your graphic description, and thanks, Carl, for making your shark tooth point.

[non-remanié]

In NJ i would say there are too many other factors, and too few "large" fossils to really make use of the facts about size sorting. Any gravel bar here is usually of sufficient cobble size to harbor a decent size NJ fossil. Most of the large cobble size gravel bars don't change much even after substantial floodwaters run over then. If the surface is barely changed, there is little chance of finding something newly exposed on it. Also there's the fact that if the gravel bar is always there, then people are constantly walking over it abd checking it because its the easiest access, so the few things that do get dropped out of the flow on it are much more likely to be found because they remain obvious on the surface, and dont get buried, and get the most eyes scanning them. I have surface collected a while in NJ and don't really care to look at many of the largest gravel bars for more than a passing glance. My favorite gravel bars are spots that I know are constantly changing, so every flood changes whats exposed on the surface and gives me a chance of something that was newly exposed. But even more important than that are any gravel/sand bars within close proximity to some deposit that I know produces fossils. But finding those spots is not necessarily easy...

[elcoincoin]

In france and belgium people are screening in sand pits and beaches, but i havent heard bout creek screening once.

[Carl]

Thanks, Elcoincoin! That's the kinda thing I'm looking for!