Stones in groups... a post-rain phenomenom

Why are the stones in groups after rain? 

 Have you ever noticed this? 

If your garden soil is stony, then after a while you find that the stones have arranged themselves into small groups. 

Here's an example from one of "my" gardens, left: their soil is very stony indeed, and whenever I'm there after a gap of a couple of weeks, and especially if we have had a lot of rain, this is what I find.

In this case, I know that the garden owner is not able to do any gardening herself, so I know that no-one has disturbed the soil, other than me, the birds, the wind and the weather.

 I've seen it time and time again, and I've often idly wondered why, but I've never really tried to find out what's going on, until one of my Clients spotted it in her own garden, and asked me what was going on.

I already know that small stones come to the surface in garden beds: annoyingly! No matter how many you rake off, more will always come to the surface.

This is an easy phenomenon to explain, it's exactly the same as when you shake a bowl of any dry materials, ie cake mix after you've rubbed in the fat, the lumps come to the top.

When potting up at home, I see the same thing: if you shake a tray or bucket of compost, the lumps come to the top.

And if you can imagine it, the whole planet is being “shaken” by geological stress, by gravity, by rotation, etc etc etc, so although we feel that the ground is pretty solid, it's actually being constantly shaken - in slow motion.

This explains why small stones are constantly working their way up through the soil to the surface. Nothing to do with worms pushing them up, as I heard one person say!

But why, once they are on the surface, do they congregate in small clumps? (Not because “the worms push them into heaps” as I heard a different person say...)

I did some research, asked around - yes, other people have noticed it, too - and the best answer I've found so far is that when it rains heavily, the big droplets push the smallest stones to one side or the other. They continue to be moved about by the rain, until they find themselves bumping up against a slightly larger stone, which is too big to be moved by the rain.

So, over the course of several hours/days/weeks of rain, each slightly bigger stone will accumulate a halo of smaller stones around it.

If I had all the time in the world, and enough money not to have to go out to work, I'd like to set up a research bed with cameras to take photos every day, to see just how long this actually takes - wouldn't that be interesting? And can you imagine watching the film afterwards, with the stones invisibly moving into clumps?

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