And so it proved. On the left is a riddle full of earth that has already passed through the big riddle. To the right is the same, after riddling. So there's still a lot of stones, (pea sized, mostly) and a few bits of glass. It means I have to riddle it twice, but it's worth it because the big riddle also breaks up any clods and separates out most of the perennial roots still lurking in the soil.
Below you can see the big riddle, v1.1; (v1.2 has a wheelbarrow to catch the bigger stones and shards of glass - it was tiresome to gather them all up for use in the rubble drain).
And below's a photo of 2 heaps of earth. The foreground one is after passing through the big riddle. I took that through the hand riddle, it took at least 2 hours, and added it to the hand riddled heap at the back, which has of course almost doubled in size now: it's about 1m high and 2m diameter at the base, which if my secondary school geometry is ok means 4 cubic metres of beautiful riddle earth. "Tilth" is the word that keeps me going.
I put the big riddle at the back there, but it's better where it was originally. I've started on the next heap now, a barrow full. On my next visit, I'll barrow all of the "extra" earth closer, so that I can shovel it directly into the big riddle, and keep it covered with a tarp so that it stays dry enough to be worked. This last point is important, I don't know when exactly I can spare time for this job, (and I'd estimate it's another 12-15 hours), and the Winter rains can't be far away now.
And this is just the "extra" earth which had accumulated in and around the Old Greenhouse Foundations. My theory is that at some time, perhaps as long ago at the 60s, the plot was left vacant, and the greenhouse's glass smashed. It lay there for maybe 20 years until the Predecessor took on the plot. By then, the level of earth had risen by means of the growth and decay of annual weeds, but this new earth was infested by glass. This theory does not explain, however, how it came to be also infested with rubble and gravel.
I'm probably about 25% into this extra earth task. Eventually, I need to give all of the beds the same treatment. When I've got the entire cultivated parts of the plot down to a spit's depth reasonably free of stones, glass and clods, then I can begin to think of moving to no-dig. A good tilth is what's needed.