Mystery


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I have no idea who to ask this to, and not a clue what it would be classified as - which is why I'm asking. If anyone has any theories, I'd be interested in hearing them.

I've lived at this property for about 25 years. When I moved in my backyard was mostly flat.

20 years ago while mowing I noticed a small incline/hill that appeared out of no where. It was just 1-2" above the rest of the ground and less than 6" in diameter.

Every year, this "hill" has grown both in height and diameter. I'm now looking at a 10-12" tall by 6-8' diameter hill in my yard, and every year it only gets bigger and taller.

I've heard of tree roots dying, I've heard of sink holes, I've had old septic systems collapse before (elsewhere, not here) - but I've never heard of hills forming out of nothing.

No trees anywhere near this spot. No activity ever done near this spot (above or under ground). No structures near this spot.

So my question is, what on earth could cause the ground to raise up like this?
 
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Does the grass on top of this hill grow the same as in surrounding areas
 
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Does the grass on top of this hill grow the same as in surrounding areas

Indeed. It's a lot shorter and often times more yellow than it's surroundings but that is because when I mow, the elevation change comes up so quickly that I often don't lift my blades high enough or quick enough to compensate, so I end up mowing closer to the ground (and often enough, strike the ground with the blades/deck). But yes - normally no dead spots or anything that sticks out.
 
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In many areas around San Antonio there are instances of this very thing. Here it is caused by a very large rock coming up towards the surface, but here the grass doesn't grow quite as thick or as healthy because the roots can't grow deep enough. Have you tried digging a hole with a post hole digger and see what is down about 2 feet deep? Here the rock usually about 4-6 inches deep and it can be huge.
 
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In many areas around San Antonio there are instances of this very thing. Here it is caused by a very large rock coming up towards the surface, but here the grass doesn't grow quite as thick or as healthy because the roots can't grow deep enough. Have you tried digging a hole with a post hole digger and see what is down about 2 feet deep? Here the rock usually about 4-6 inches deep and it can be huge.

I haven't recently. I was curious about 6-8 years ago when neighbors mentioned it in passing (they thought it was strange as well) and I took one of those 3' ground anchors I use to hold down the kid's trampoline and had no problem at the time pushing it straight down. As I recall, there were no roots or any big rocks or anything that came up when I twisted it in ...some little rocks, but we got plenty of them around here in the ground (1", maybe at most 2")..

I think I got some 6' rebar in the shed, could hammer some of that down there and see if I hit anything but I somehow doubt I will make contact with anything.

Rocks does make sense though .. but it would have to be a massive one.. best explanation I've heard so far.
 
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I haven't recently. I was curious about 6-8 years ago when neighbors mentioned it in passing (they thought it was strange as well) and I took one of those 3' ground anchors I use to hold down the kid's trampoline and had no problem at the time pushing it straight down. As I recall, there were no roots or any big rocks or anything that came up when I twisted it in ...some little rocks, but we got plenty of them around here in the ground (1", maybe at most 2")..

I think I got some 6' rebar in the shed, could hammer some of that down there and see if I hit anything but I somehow doubt I will make contact with anything.

Rocks does make sense though .. but it would have to be a massive one.. best explanation I've heard so far.
Worst case is that it could be the actual bed rock shifting. Are there any concrete foundations failing nearby?
 
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Worst case is that it could be the actual bed rock shifting. Are there any concrete foundations failing nearby?

Not that I'm aware of.

In case it helps, my fresh water well sits about 30' from this location ... the well is 50' deep. The nearest structure to this area is a neighbors property. Several years ago they added onto the house and ended up spending 30% more due to unexpected ground conditions (they added onto their basement as well, which meant digging down over 12' I guess). From what I was told, they hit a lot of sand and clay which wasn't expected, which meant more materials required for the foundation/basement they put on.
 
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Not that I'm aware of.

In case it helps, my fresh water well sits about 30' from this location ... the well is 50' deep. The nearest structure to this area is a neighbors property. Several years ago they added onto the house and ended up spending 30% more due to unexpected ground conditions (they added onto their basement as well, which meant digging down over 12' I guess). From what I was told, they hit a lot of sand and clay which wasn't expected, which meant more materials required for the foundation/basement they put on.
Very interesting. Any fluctuations in your well water level? BTW where are you located?
 
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This is in central Wisconsin - the area is mostly very flat and unremarkable.

The water table today is a little lower than usual but we had a very mild winter with not much snow. No one to my knowledge in this area has ever had any issue with their well water running dry .. even many years ago when all of the grass was dead and brown, and we hadn't gotten rain in months, there was enough flow to sustain the neighborhood.

The water itself is what you would expect from this area (hard water) and hasn't ever changed since I moved here (I test it monthly to fine tune my water softener when needed).

I agree, it's pretty strange. It doesn't bother me, it is annoying to mow over to be sure, but it is what it is. I'm more curious than anything.

Maybe I'll get lucky and be the first one in Wisconsin to have private tours of a mini-volcano in my backyard... or.. it's just a damn rock.
 
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This is in central Wisconsin - the area is mostly very flat and unremarkable.

The water table today is a little lower than usual but we had a very mild winter with not much snow. No one to my knowledge in this area has ever had any issue with their well water running dry .. even many years ago when all of the grass was dead and brown, and we hadn't gotten rain in months, there was enough flow to sustain the neighborhood.

The water itself is what you would expect from this area (hard water) and hasn't ever changed since I moved here (I test it monthly to fine tune my water softener when needed).

I agree, it's pretty strange. It doesn't bother me, it is annoying to mow over to be sure, but it is what it is. I'm more curious than anything.

Maybe I'll get lucky and be the first one in Wisconsin to have private tours of a mini-volcano in my backyard... or.. it's just a damn rock.
You mentioned sand and clay. Doesn't a sink hole begin with underground water erosion? Erode away the sand and that leaves clay. And water pressure will stretch clay like water in a balloon. Wouldn't it be something if it was an artesian spring forming and hopefully not a sink hole. It might be fun to talk to a hydrologist about this.
 
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zigs

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Welcome to the forum :)

My first thought was a salt dome

https://geology.com/stories/13/salt-domes/

But I just checked where Wisconsin is and it doesn't seem to be in the right area for it.

I like Chuck's idea of the boulder working it's way up :)

The other thing to consider would be, is it actually a hill rising or is the ground around it shrinking/falling?

I've seen things apparently rise up on peat lands ( I used to live in Somerset, a lot of peat around on the levels) because the peat is shrinking as it dries up through man made drainage of the farmlands. Some of the roads are like driving over a roller coaster and some houses need steps to get up to the front doors now, even though they were originally built at ground level.
 
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I`m not clever enough to comment on all this stuff, but just wanted to say ...Welcome to the forums deklar (y)
 
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Welcome to the forum.

As I am not familiar with soil and growing conditions etc in the US. Might I ask. Have you actually attempted to find out if something is forcing it's way up. There are of course ground/soil dwellers that do push up mounds of subsoil Worms, moles and other creatures. Please have a look at the link. https://www.sciencedaily.com/releases/2018/08/180801131607.htm

Best wishes.
 
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A method for checking or rather comparing water content of different areas of your soil would be to compare a relatively undisturbed volume of soil to the same volume of water. The weight of water would be the standard and would be necessary to determine and compare the specific gravity of the soil sample to that of water. A dried sample could or would be compared as well. I would think a very fine clay capable of swelling would have to be imported to that area, but it is strangely not out of the question. Yazoo clays are exported from Mississippi to seal pond bottoms and they swell enough to make humps in the interstate roadways of MS.
 
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A soil sample from the area in question and samples of the surrounding area may prove to be interesting.
 

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I had an underground ant's nest. As they excavated they dumped tiny grains of soil on the surface which were not visible in the grass but gradually made a lump. It would have to be a big nest to do that though, still everything is bigger in America, right? ;) If it were me I would be tempted to lift the turf, dig it out a bit, and put the turf back level. It would make mowing easier and you would get a look at the top few inches. Knowing my luck though it would then probably sink again. Sorry I don't have any really good suggestions, but please keep us posted, this is fascinating.
 
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