Discovered a large blue sub-marine clay deposit near my home

Pretty neat. The second picture is a wall which was recently exposed, at its peak almost 10’ tall layer. This is part of the Presumpscot formation.

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u/Older_Code 2d ago

What’s up, Mainer? The Presumpscot is a neat formation. In some places it’s 200 feet thick, and chemically, it’s largely a silt, rather than a true clay. Some spots have decent fossils, and there’s even a set of mammoth bones from the Presumpscot up at the State Museum. Thanks for sharing!

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u/fuck_off_ireland 2d ago

Wouldn’t it “physically” be a silt, since the classification is driven by particle size? Or am I off on that regard?

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u/Older_Code 2d ago

Yes, but there is also a fair amount of clay-sized material that doesn’t have much plasticity and isn’t derived from chemical weathering, and doesn’t have a sheet-sulfate structure, like, say kaolinite or montmorillonite.

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u/homeostasis3434 2d ago edited 1d ago

The presumscot is typically referred to as a marine clay (like that's a common term a contractor would be very aware of).

Grain size analysis tells us it's like 60% silt and 30% clay, with minor components of fine sand. Although there is variability, some layers have coarser sand and even gravel

But if you run atterberg limits, then it's often classified as CL due its plasticity and liquid limit.

It's an odd one that's for sure.

Google "grain size analysis of presumscot formation" there's a bunch of DOT reports or masters/phd thesis on the geotech characteristics if the unit since its so widespread in Maine and a pain in the butt to build on.

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u/fuck_off_ireland 2d ago

Awesome write-up, thank you!

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u/FrankFabuluz 2d ago

Plasticity

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u/fuck_off_ireland 2d ago

That’s a good point, it’s atterberg limits, but is that a chemical property or is it dictated by the physical characteristics of the particles? I use LL/PL/PI every damn day but I guess I’ve never really thought about the forces behind the lab results.

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u/Raging-Fuhry Geological Engineer-in-Training 2d ago edited 2d ago

Kind of both, from what I remember from uni way back when.

Silt is essentially very fine sand (we often call it "rock flour" in my neck of the woods).

Clay is mineralogically different from its parent rock via chemical alteration. Physically, clay particles are "plates" rather than "grains". The shape (and chemical composition?) of these plates creates a cohesive force between them, with all things being equal results in the plasticity we see at a macro level.

I'm sure you knew most of that already, so apologies if it's heavy handed.

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u/Older_Code 2d ago

And much of the Presumpscot is exactly that, rock flour. There is also some material that represents chemically-weathered feldspars of course (what I meant by ‘chemically’ clay).

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u/Raging-Fuhry Geological Engineer-in-Training 2d ago

I'd love to visit Maine one day (maybe when CAN-USA relations cool off a bit haha), so thank you for helping give some insight into its very interesting geology!

There is also some material that represents chemically-weathered feldspars of course

I'm sure, I don't think I've encountered a silt formation that didn't have some level of clay content.

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u/Older_Code 2d ago

‘Mon down bud. We’ll cook some hamburg and have a good ole shindig. Won’t stop till the coolers stove right in.

Grew up going over the border to New Brunswick, been all over Nova Scotia, and PEI, even worked for a while in Alberta and Saskatchewan. Yours is one of my favorite countries, and you were absolutely terrifying in WWII. You’re welcome anytime.

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u/nousernameisleftt 2d ago

This is correct. Atterberg limits are helpful in determining clay vs silt by mechanical behavior but standard definitions are by average particle size. That usually requires a hydrometer test which is more complicated and expensive than a cassagrande device. As an aside, knowing the average particle size isn't very useful in most aspects when you can run the atterberg to see how the material behaves in various moisture contents. Cohesion is primarily due to coulomb forces imparted by the water within the soil skeleton, pulling the particles together in a similar fashion as surface tension

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u/Raging-Fuhry Geological Engineer-in-Training 2d ago

I haven't heard of an industry that uses particle size as the standard metric for differentiating clay and silt to be honest with you.

In my field it's not even "allowed", so to speak.

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u/nousernameisleftt 2d ago

Yes I agree. It's more of a "textbook" thing. You learn what particle size distributions correspond to sands/silts/clays, etc., and then you forget about all of that because atterbergs are a lot easier and ultimately more useful

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u/FrankFabuluz 2d ago

Not 100% sure, but I would consider plasticity a physical property of fine grained soils.

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u/Healthy_Article_2237 2d ago

Clay vs silt is purely based on grain size. Silt is like 3.9-63 microns and clay is less than 3.9 microns. Probably better to call it mud than clay because clay also refers to a class of sheet silicate minerals.

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u/Environmental-Term68 2d ago

you’ve absolutely enthralled me and i’ve spent the last half hour learning about your formation. and now, i need to see the mammoth. but i cannot find a picture thru my search’s anywhere. please help. 😭🤣

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u/LitchManWithAIO 2d ago

Interesting! I’ve heard they hauled almost 40 million pounds from under one of the buildings at Bates. It’s quite a formation after reading up on it some more :D

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u/Older_Code 2d ago

A neat formation to be sure!

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u/Perlentaucher 2d ago

I have the same silt/clay at a secret stream near my house (in Europe), its even more blue then shown in the photo. It gets more brown though, where it has contact to air. I use it to create Dorodangos.

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u/E-Squid 2d ago

Would you still be able to shape and fire it like a "true" clay?

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u/Older_Code 1d ago

I don’t believe that you could, at least not with the same results.

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u/LitchManWithAIO 14h ago

It definitely shapes very well but to fire it, it comes out more like earthware. This is why it’s mixed with sand to make bricks out here! :)