Any gardener will tell you that they find rocks no matter how many times they till up the dirt and plant. The very first summer I got two bucket fulls of rocks after I tilled and raked out the dirt. Every summer afterward I find more rocks from the exact, same spot of soil. Summer after summer. When I’m 98 years old, I’d bet there will be a few rocks in a pile after I’ve planted that summers garden (my 68th garden in that spot by then).
So, where are they coming from? When you rake with a dirt rake, any big stones get caught in the tines and easily heard clanging against the steel tines & found. It seems very unlikely gardeners are missing that many large rocks year after year. They got to be coming from somewhere.
Do rocks move around and migrate upwards through the soil?
Triva: New England farmers traditionally heaped any rocks they found during plowing into piles to mark their property lines. Some of those piles extended into fences. Hundreds of years have covered these stone piled, fences in moss and vines. Today they add a very bucolic atmosphere to the New England countryside.
Imagine the frustrated farmer trying to plow this field. 
While rocks can move upward (uplift,) it’s probably more common, especially in agricultural areas and New England, for soil to erode away, exposing rocks that have sat in the same place.
Wind and water carry out a lot of erosion, and after a big snowmelt it can be hard to tell how much soil has eroded away with the water. Retaining topsoil has become a major concern for farmers in a lot of areas, although I’m not sure how important it is in New England.
Are you in an area where the ground freezes? I’ve read that each time the ground freezes it squeezes the rocks up a little bit higher.
For a dramatic illustration of the erosion process Wevets describes, see desert pavement: Desert pavement - Wikipedia
I doubt that’s what’s happening here, though, since you would presumably have noticed if your garden was receding into the ground due to large amounts of erosion. I think what’s happening is that you’re not really catching all the rocks in your topsoil each year and it’s just chance whether your tilling leaves the rocks in a position that you can find them with your rake. I’d bet that if you took the top foot or so of top soil, ran it all through a sieve and then put it back you wouldn’t find any more rocks.
They could have been carried by swallows.
African or European?
But more seriously, is it the rocks coming up, or the soil shifting down?
rocks float. happens especially with repeated ground freezing.
the peanuts always end on the bottom in a can of mixed nuts.
Really? With freezing, don’t they rather get preferentially pushed up? Due to their larger surface area? Grains of sand and clay will get pushed to one side, but a stone will be more likely to get pushed up.
We had rocks moving up through the asphalt of our driveway, and it had to be removed, graded and resurfaced. It was 35 years old, and presumably, it didn’t have rocks poking through it when it was laid. New Hampshire.
Go buy a big can of mixed nuts (not more than 60% peanuts). Open it. Give it a little shake. You will see that the Brazil nuts, the pecans, the cashews have come to the surface. When you eat your way through the can you will find that the last few handfuls are mostly peanuts, hazelnuts and broken pieces. What is going on? As the can is jostled the smaller nuts slip into the voids and push the bigger nuts to the surface.
The same thing happens to the soil. The fine dirt particles gravitate to the voids and push the larger particles (rocks) toward the surface. The jostling is provided by freeze and thaw cycles and constant little earth tremors that we can’t even feel.
One of the horrors of growing up in the country was the spring ritual of “picking rocks,” walking the plow ground to pick up rocks and tote them to a stone boat to be dumped in some untillable corner.
Not living in an area that freezes, I forgot about frost heave, the process filmore and others have mentioned that involves differential ice formation in the soil overturning and remixing soil and rocks. Likely to be important in areas with winters like New England, and even in tropical alpine areas.
Now there’s a blast from the past! Working on the stone boat! Best done on a cool spring day before the summer heat and humidity sets in.
As far as I can tell, the same thing happens in any soil that is exposed to vibration. The soil ‘sorts’ itself over the course of time with larger particles (as in gravel or rocks) moving gradually higher in the mix. This can happen even when the soil is not regularly subjected to subsurface ice as was the case this past winter in East Tennessee. There are many small rocks on the paths that were below the surface just a year ago. This isn’t erosion causing this phenomenon, in that the average level of the soil remains relatively constant. However, when the larger particle are removed, the remaining soil does compact down. This may lead to the impression of erosion and I suppose if you count the effects of people moving rocks from the path, then it is erosion of a sorts.
Anyway, just chiming in after working on picking up rocks that magically showed up in the last year.
I’ve walked the fields picking rocks a few too many times myself. Granddad had a stone boat that we hitched to his mule to move.
My garden is small enough that a typical 20 gal yard waste container is all I use for rocks. They have a wide enough top that I can usually pitch the rocks into it from wherever I’m standing.
A city slicker is watching a Maine farmer putting rocks on the rock boat in the spring. As the farmer approaches the road where city slicker is watching from he speaks up.
City Slicker: Where did all those rocks come from?
Farmer: The glacier brought 'em.
CS: Where did the glacier go?
F: Back for more rocks.
It didn’t really happen noticeably where I originally came from so other people may have a time believing it as well but I too can attest that new rocks (big ones too) show up year after year in New England. It is especially bad in Northern New England. It isn’t a case of people missing big rocks year after year for 200 years or the soil eroding away. New ones just show up every spring.
I have had the following conversation with people more than once:
Them: I grew up on a farm in New Hampshire
Me: Oh really, what crops did you raise.
Them: Rocks mostly but other stuff sometimes too if we were lucky.
It’s not the rocks floating up. It’s the dirt sinking down. When the ground moves and opens small gaps, it’s the smaller particles that fill in the gap. As a result of the smaller particles like dirt and pebbles moving downward, bigger particles like rocks get exposed on the surface.
i think the mechanism of smaller particles filling voids under larger supported objects gradually allows the smaller particles to sink while the larger object doesn’t.
in winter frost might heave objects near and penetrating the surface. as the soil expands, when freezing with water content, it tangentially pushes objects up. this would act on lifting exposed rocks.
since freezing is from the top down, there is an ice cap above any subsurface rocks. so i don’t think heaving is at work here. i think the action takes place during the thaw. the soil thaws from the top and bottom but more from the top. this results in a lot of wet soil and standing water in low spots on top of an ice layer. soil around a rock would be wet and able to move. a rock may stand on a chunk of ice, where the nearby soil at the same depth be thawed. this soil might then have lots of chance to move under the still supported rock. eventually the large rocks float
i think float is used metaphorically.
I don’t think it is erosion. How many have you have hit a rock with the lawnmower in the middle of a lawn that has had a good stand of grass for years and the same old mower? No, each year it is a little higher and eventually you hit it unless you have noticed it and removed it.
To float, rocks would have to have an spg less than the soil. Both are highly variable. I found figures on some rocks of about 2.5, but nothing on soils. Do we have any data to suggest heavier rocks sink to bedrock and it is only the lighter ones that rise?
May be more than freeze/thaw too. Surely in the 10,000 years since the last ice age most of the ones above the frost line would have made it to the top. A couple of ideas that may or may not be right, but to think about.
I’m hoping what you mean is that it’s not always erosion. Whether it’s erosion or not depends on local topography, elevation, and climate.
Case in point: http://www.flickr.com/photos/30973257@N03/7140703663/
Tell me if you think that rock wasn’t exposed by erosion. If you have a good reason to think it wasn’t, I’ll tell you where the relevant topographical features near the rock are - I’d be shocked if you think there’s a good reason to believe it wasn’t erosion when I know where the picture was taken and you don’t.