Argillite

[CMD]

Hoss wrote:

Charlie they have some but igly material in your neck o the woods but they sure did know how to make a nice point out of some real crappy material.
That Vosburg looking point in the first picture is some rough material but that point overall is a beauty.

  Thanks, Hoss. Helen found that Vosburg. 2 3/4" and pretty thin at 5mm max. Actually has/had decent edge work but water weathering makes it impossible to see in the photo.  Anything other then quartz or argillite is a good day around here.  Nice points are possible with both, more so quartz of course, but we need a few beaches like the one Chris hunts

[pkfrey]

Charlie, Did you get my last reply email from the email I received from admin@? Thanks, Paul

[painshill]

Cliff makes a very good point about archaeologists versus geologists in terms of their ability to put the right names to rocks and Charlie makes some very good points about the way in which New England deposits differ from those in say New Jersey. It’s worth remembering that every rock belongs to a family within which the distinctions between family members are often blurred. Geologists frequently try to simplify the terminology for those outside their community in a manner that’s probably unhelpful to archaeologists interested in tracing the specific origins of rocks.

The USGS uses the following simplified definition for argillite: “a compact rock derived either from mudstone or shale, that has undergone a somewhat higher degree of induration than mudstone or shale but is less clearly laminated than shale and without its fissility, and that lacks the cleavage distinctive of slate.”

The use of phrases such as “somewhat higher induration” and “less clearly laminated” imply (correctly) that the dividing line between argillite and rocks that should be given other names is not distinct. But there are several problems with that simplified definition as it stands.

“Induration” is the hardening of a rock by heat or baking but is also the hardening of a sediment through cementation or compaction, or both, without the introduction of heat. Those are vastly different geological processes in terms of the end result. Metamorhposis is a form of induration, but there comes a point where you no longer have a sedimentary rock and - by definition - you no longer have argillite. Argillite ultimately metamorphoses to slate, phyllite and a rock known as pelitic schist. Partial metamorphosis leaves you with a problem about what you’re going to call the end result, but in most cases the proper term for what many people call argillite would be “argillaceous slate” – especially in New England.

Then we have particle size to consider. Argillite is characterised geologically by being formed from high proportions of fine, silt-sized particles. The most extensive argillaceous deposits in New England are in Massachusetts and these were mostly formed by the relatively rapid flooding of aqueous sediments into a large rift valley. So, the true argillite tends to have formed from small silt-sized particles in the uppermost regions of the strata. By contrast, in New Jersey, the sediments were mostly deposited cyclically over long periods of time and much of the sediment was wind-blown. That means the deposition layers are frequently as laminated as shale, contain larger sand particles and, in some places, trend towards sandstone or quartzite. In most cases, the proper term for what many call argillite would be “argillaceous shale” or “argillaceous sandstone”

Finally, the USGS definition makes no reference to rock chemistry (ie composition). What properly characterises argillite is that the clay/mud oozes and sediments from which it forms are high in silica, but the particles themselves are not bound by silica… that’s what differentiates it from shale. Also, it’s typically high in aluminium, but a lot of so-called argillite doesn’t meet that criteria… and you can’t tell just by looking. It also has variable amounts of (principally alkali cation) impurities which determine the colour. That can be grey-green to green, purple to reddish, brown to tan or black to grey.

All things considered, it would be better to refer to “argillaceous rocks” to avoid the confusion between so-called “argillites” that have very different properties, some of which may not meet the more detailed criteria beyond the simplified USGS definition.

[kayakaddict]

I enjoyed that, thanks painshill.  Is there a good USGS site that gives even more on this?  I'll go look and see.  BTW I always considered NJ to be part of New England.  Maybe I better check an old school social studies book.  One more thing, it might be a nice project for someone to archive a nice variety of argillacious rock photos from different areas.  Examples of different stages of wear, etc.  Is there a dissertation out there somewhere online about this?  Anyone?  I love that crude rock.  The first 5 or more times I found a point it was an argillacious rock.

[painshill]

You're welcom kaykaddict.

I took the Wiki definition for New England as being 6 states - Maine, New Hampshire, Vermont, Massachusetts, Rhode Island & Connecticut.

The USGS website is here… it’s excellent and I use it frequently:

http://www.usgs.gov/

…and the entry for Geologic units containing argillite is here:
http://tin.er.usgs.gov/geology/state...text=argillite

I'm not aware of a pan-American study of argillites. Most of what I have in my database has been downloaded from multiple sources and pieced together.

[CMD]

pkfrey wrote:

Charlie, Did you get my last reply email from the email I received from admin@? Thanks, Paul

  Hi Paul, I don't see anything in my emails or PM.  I do think you will be an excellent mentor for new members from the Northeast.  I respect your knowledge of this region, Paul.

[CMD]

Roger, thank you for the clarification.  I'd always just assumed the argillite from further south was also argillite, just different in appearances, being much grainier. Archaeology is interdisciplinary, often depending on chemistry and geology to trace raw materials and particular lithics back to their source quarry if possible. Maybe the geochemists go easy on the archaeologists and simplify names, I wouldn't know.  I know the few archaeologists I've been acquainted with here seem to know the basic rock types of our lithics. For the most part, they know far more then this collector. Felsite is a common lithic here. I can't name them by location simply by looking at them(for example, I could not recognize Blue Hill felsite enough to say anything more then it's felsite). So I was just defending my area archaeologists because I don't believe they're entirely ignorant of our regional geology.  But, I want to thank you for, once again, pulling out all the stops and clarifying the "problem"  so that we understand the differences. Your the best, Roger
BTW, I'm saving this thread, as by my count, this is the 4th time the misunderstanding regarding argillite has cropped up in threads I've started on the Internet, not just here.  Roger, your clarification will now serve me well if needed in the future.

[CMD]

[QUOTE]CMD wrote:

Charlie, Did you get my last reply email from the email I received from admin@? Thanks, Paul

  Hi Paul, I don't see anything in my emails or PM.  I do think you will be an excellent mentor for new members from the Northeast.  I respect your knowledge of this region, Paul.
  Hi Paul.  Turns out I got your pm, but at the moment, cannot reply with either pc or iPad. So, I asked someone to send you my email if they are able. Looking forward to working with you in the future

[CMD]

Considering the category is "Learn about material types",  I'd say we did just that ,with examples from Pa., NJ, Ma., and RI.

[edharrow]

I have not seen an example of New England argillite to compare to New Jersey's, but your description of it is spot on.  From what I've seen on this forum, Many of New England's argillite pieces seem to be in excellent condition, while most of the examples from New Jersey are HEAVILY weathered and barely resemble points at all.  However, a few rare examples (of which I do not have photos of at this time) are perfectly preserved, and do not exhibit flaking of any kind.  It is my belief that pressure flaking was used only to rough out the size, and any finishing work involved grinding due to the fragile properties of the material.  The surface of the heavily weathered points does look like sandstone, with angular 'steps' seeming to be missing from the pieces.  If I can find an example of a ground argillaceous sandstone point I will surely post it here.  And big thanks to Roger for the explanation!

[Sundancer73]

CMD wrote:

Most collectors are somewhat familiar with argillite. It was used as a lithic in many parts of the country. Here in southern New England, it is the second most common lithic after quartz. Slaty, sometimes more of a siltstone, most would agree it is quite a poor choice for use in flaked stone tools. It usually works poorly, and the results are often crude. Throw in water weathering on a shore, and you get the picture. But, sometimes argillite can be nicely flaked, as seen in the first point on the left. The little triangle 3rd from left may be an early triangle. 2nd from left is a Vosberg, and last from left an Orient Fishtail. First was not a personal find, it was found in Dighton, Ma. The others are beach finds and all would be above average for this rough material.
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A good friend brought me some Argillite from New Zealand last summer when he came to USA to scuba dive here. It's almost identical. He's an archaeologist there and says the Maori have been making adzes and blades from it for centuries.
Feel free to share argillite from elsewhere!

[turkeytail]

Osceola Co. Michigan found by Elias Keragis.

[painshill]

Apologies that this old thread has come to the top of the list. I corrected some picture errors which shouldn't have had that effect... but it has randomly happened a couple of times now for unexplained reasons.

[TSUMS]

I find alot of green colored argillite points here in new england..