So - the mystery of the Spring flints have been solved! Pippa explained it to me at the Lynne Johnson workshop yesterday, and, even though I got her to repeat it twice, because I was obsessing about my imminent haircut I didn't really take it in.
I looked up "How rocks appear in your garden" though (carefully avoiding the use of flintstones which just kept coming up with Yaba-yaba-doo) and an Indiana Public Media site put it very well:
Where The Rocks Grow
Any place that has winters cold enough to freeze the ground might experience the magical appearance of rocks welling up from beneath the surface. This is so common in the eastern U.S. that the rocks are called “New England potatoes.”
Here’s what makes these stones mysteriously appear. Stones are better conductors of heat than soil, so the stone conducts heat away from the warmer soil beneath it. That colder soil under the rock then freezes before other dirt at the same depth.
Remember that when water freezes it expands. So, when the water in the soil under the rock freezes, it expands and pushes the rock up a little.
When the ground thaws a space is left under the stone which fills with dirt, so the stone rests a little higher. Over a period of time this repeated freezing, expanding, upward push, and filling underneath eventually shoves the rock to the surface.
So, these are our East Garston Potatoes!
And here's a not-very-good photo of Pippa's really beautiful Baltimore