I meant to ask: why put the clay on the bottom?
I'm just speculating, but since it tends to wash out of soil (elluviate downwards) wouldn't it be better to make it the top layer, or better to mix it somewhat uniformly?
Then as it washes out of the top layer over a few years, it will "even out" the soil structure.
Where I live, "everything is clay" and "drainage stinks". Thus I need my lower layers to drain as well as I can get them to. I set up one bed with deeply amended uniform soil, and in just one year I found that enough clay migrated downwards that everything below the top 6-8" was once again gooey, poorly aerated clay.
