An Ohio Geo-Puzzle

A post by Anne JeffersonI had the good fortune of going out in the woods a few days ago with colleagues here at Kent State University. We were in a second growth forest, probably fairly typical for this part of northeastern Ohio. The upland forest had lots of maple trees, and the bottomland forests had cottonwood and sycamore. The forest is underlain by many meters of till (with silicic clasts) and below that are various sedimentary rocks. I was there to take a look at some small streams and wetlands as potential field and teaching sites. Towards the end of our tour, my colleague brought us past this site:

My first view of the geo-mystery

My second view of the geo-mystery

My colleague described the site as the ruins of a “sugar shack”, which I connected with the maple trees to mean that this was the foundation of a small-scale maple syrup or sugar production facility.

But what really caught my eye were the tabular black rocks, which seemed completely out of character for the region.

Close-up of the black rocks. Wading boot for scale.

So, I know what the black rocks are and I have a pretty good idea of why they are there, but I don’t know where they originated. I’d like to hear from our readers what they know or can deduce about these mysterious black rocks of northeastern Ohio, so share your thinking in the comments. I bet together we can get to pretty good story of the human history of these geopuzzling erratics.

Categories: by Anne, geopuzzling

Comments (18)

  1. Lockwood says:

    Okay, thoughts… based on last pic (1st two really not helpful), looks like vesicular basalt. I was thinking it *might* be slate, brought in for roofing. However, it’s much too thick and chunky for that, and it would be a lot of work and money for a sugar shack (which, yes, is an old-timey term for a maple syrup/sugar building… don’t miss Chardon’s maple festival! ) It looks like there’s numerous chunks of the same stuff, which is a little puzzling, if it’s a glacial erratic. So my guess is that a single large erratic was split and used as a component of building stone, perhaps as a chimney for the fires used to boil the maple sap into syrup. The source must have been Canada- there’s no native basalt exposed in Ohio. I think you’re a bit far east for this to have come from failed rift of MI Copper Penninsula/SW ON, and my ON geo is rusty enough, I can’t think of likely source for subaerial (which is most likely envt, based on vesicles) basalt in that province.

    Another, albeit unlikely, possibility is that it’s slag; there was a fair amount of bog iron smelting around eastern Ohio in early settlement days. I think that’s unlikely because 1, most Ohio slag is more colorful, 2, these examples don’t look as glassy as I would expect, 3, this looks like an upland area, and bog iron, as the name implies, is a wetland product, and 4, there are generally at least some remnants of a furnace where they once were. However, that vesicular texture is common in slag, and based on photos and the sparse info you provide, I can’t discount that possibility outright.

  2. Matt Herod says:

    Hmmm. Tricky. The erratic idea seems plausible except I would expect more rounding if it were transported glacially. You’re right Lockwood, there is not much basalt in Ontario. Lots of limestone and shale in my area and then Canadian shield which is metamorphic and intrusive igneous…nothing volcanic in origin nearby.

    However, when I zoom in a bit I do see what appear to be vesicles. Could be amygdular basalt with the amygdules weathered out? There is a place in Quebec that has them. Looking forward to the solution!

  3. Anne Jefferson says:

    Note that you can get very large versions of the images by clicking on them.

  4. Lockwood says:

    I had noticed that, Anne. But I just went back and noticed in 2nd photo (slightly upper left of middle) it looks like black rock was used as floor/pavement of building. And more I look at #3, more convinced of basalt. To left and above of boot toe, vesicles clearly have “preference” to the left of the rock, which may be a strat “up” indicator.

  5. Ron Schott says:

    I’d also be skeptical of whether you could get Keweenawan basalt this far east in Ohio via glacial drift, but it’s not entirely impossible if it originated along the east shore of Lake Superior (I know Keweenawan-aged basalts in outcrop there) and came down via the Lake Huron lobe of the Wisconsinan ice sheet. There’s still quite a ways to go to get it across to the east side of the state, but the first part of that journey is at least plausible.

  6. Lab Lemming says:

    Are they part of the ruins?

  7. Howard says:

    I’m not convinced that these are basalt. I think the black colour is secondary. I’d guess that these rocks formed the base or floor of a stove/furnace/firebox that was used to boil down the maple sap. Some of the “vesicles” resemble weathered-out rip-up clasts that one often sees in sandstones, or even limestones, which is what this could be. The black colour would be expected from years (decades?) of burning wood, no doubt with syrup dripping down from the cauldrons and being reduced to a very persistent layer of carbon.

  8. Lab Lemming says:

    I’m going to guess that they are slag byproduct from historical East OH/west PA steelmaking that has been reused as a building material. If you picked one up, you would note that slag is a lot less dense than basalt, even if there are a few iron blebs left in it…

  9. Andrew Alden says:

    Your description is prejudicial. You call them “rocks” and “erratics” without strong evidence. To me the dull dark color, absence of crystals and irregular vesicles strongly suggest slag, although the pieces are large. I see that Lab Lemming, posting as I write this, thinks so too.

    Slag and brick are appropriate materials for a sugaring shack. The limestone and shale making up this part of America would not be.

  10. Lab Lemming says:

    While we’re being pedantic, I see a lot of beech trees and beech leaves in the litter, but not much maple…

  11. All right, for the pedants in the crowd :-), here are a few clarifications (hints?)

    re: tree cover. I had noted that the trees right around the shack were not maple, but when the ecologist (with the long term tree plots on site) leading the trip tells me that maples are abundant in the woods, I’m inclined to believe him.

    re: slag v. basalt. I’ll admit that I did not consider the possibility that these could be slag, because from the first glimpse of them, and supported by the closer look I gave them (though I had no hand lens on me), my geologist brain told me they were basalts. No, perhaps you could argue that I am a basalt-o-phile inclined to see lava even where I should see slag, but the counter-argument is that having spent 5 years working amidst basalts I’ve got a reasonably well-trained eye for IDing them from hand sample. However, if the slag theory persists, I’ll see if I can lug a sample back with me should I get back to that field site in the near-ish future.

    re: erratics. I did not mean to imply a particular mode of transport with my choice of that word. I merely used it to imply “a piece of rock that differs in composition, shape, etc., from the rock surrounding it, having been transported from its place of origin.”

    All right, hopefully that clears up any confusion. Back at it!

  12. Laurel says:

    I don’t know enough about geology to even venture a guess, but I’m very curious about the answer, and im also curious to know where the site is :) I haven’t noticed anything quite like that in NEO and not sure how common it was to use rocks like that as flooring, lots of sandstone in the older foundations though.

    Oh, and we always avoided the Chardon maple festival. It is total chaos, terrible traffic, no parking. It is almost like a county fair with carnival rides and everything, all crammed into the tiny Chardon square. There are many cool, smaller sugaring events though. You could check out what the Geauga parks or Lake Farmpark have to offer. These smaller, learning focused events might be more enjoyable.

  13. Lockwood says:

    @Laurel: Now that you mention it, I *do* remember parking is an issue, but my fond memories of Chardon’s Maple Festival are from the 60′s… so a bit musty, to say the least. Thanks for clarifying modern situation, though to this day the maple stirs are a subject of lust.

  14. Ann says:

    Because of all the bricks in the area, it definitely looks like it was brought into the area, and it’s my guess it was man and not the glaciers at this spot. They probably used these rocks because of their ability to hold the heat in the making of the maple syrup.

    The Cleveland/Cuyahoga county (just to the north) has long been known as a steel town, ie the place where the steel was made. They used the Cuyahoga river and the Erie canal as a means of transporting the steel and the products to make it. Seeing slag deposits in the area would not be unusual because of the ease of transporting it to those locations. My guess it is slag and not basalt.

    In the area there are limestones & sandsones that are used a lot in the construction of buildings, I’m also wondering if it might not be one of theses and it’s been altered due to it being part of a furnace.

    • Ann says:

      I’ve come back to it for a second look, and the more I look at it I wonder if those stones didn’t come from the Berea Sandstone Formation (Mississippian) or less likely the Sharon Conglomerate (Pennsylvanian) . Those are the cap rocks in northeast Ohio. The way the moss is growing on it makes me believe its more porous than it looks. (Did you pore some water on it or test it with HCL acid?)
      The Berea Sandstone has long been used as building material and was quarried in the area. The most famous building that I can think of that was built with the Berea Sandstone is Garfield’s tomb.( It does weather gray like that.
      It is also present in the Cuyahoga Valley National park ( ) The Berea Sandstone is the cap rock of the Brandywine Falls & Blue Hen Falls in the area. (“Geology of the National Parks, 6th ed” by Ann Harris, Ester Tuttle & Sherwood Tuttle (2004).

  15. Erik says:

    Get a sample! Make a thin section!

    That solves everything!

  16. Ron Schott says:

    Alright, our guesses are in and it’s been a week since the original post. What’s your take on these mystery stones, Anne? Inquiring minds want to know!

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