An Iowa Garden

Our woods lies on the southern slope of a modest hill, with a dammed pond at the bottom; a benificent and benign spot for gardening as far as the climate goes... but then there is the soil. Iowa is known for its rich, loamy black soil, but the riches are not universal. When I started digging on our land, I found a peculiar and frustrating phenomenon: there is a layer of sticky clay on top (which hardens in dry weather to something akin to asphalt); below the clay is about a foot of sand, and below that is a layer of black, finely grained soil. Since we are near the top of a hill, I couldn't figure out where all the sand came from, but was suspicious, since this is the only undeveloped land and it's at the end of a dead end street, that when all the home construction was taking place around here that this was the dumping area. This idea was bolstered by the presence of a large "puddle" of hardened concrete that was dumped on the parking at the end of the street by someone. This so-called topsoil was not what I had in mind for growing delicate, moisture-loving plants, so I've laboriously dug out this sand and clay to a depth of two feet wherever I wanted a flower bed, and refilled the hole with my own soil mix... I suppose I've hand-dug a couple of basements by now. For years I've complained to anybody who'd listen (basically Liz) about the mysterious people who dumped all this sand and clay on top of our hillside.
Yesterday, came enlightenment: at a meeting of the Johnson County Heritage Trust, the natural habitat preservation society that I belong to, a geologist was showing us some small wetlands that he'd discovered in this area that needed to be protected, and he showed us the geology that created them. Iowa's ancient geology was formed by this area being at the bottom of shallow, inland seas, but most of what we see at the surface was created by a series of glaciers that covered much of the state. The most recent glacier was the Wisconsin, which finally completely retreated only about 13,000 years ago. Most of eastern central Iowa geologically is what's called the Iowa surface; an area intensely eroded by the glaciers, leaving a region of gently rolling land, with long, low hills (called pahas) that consist of glacial till covered by wind-blown, fine glacial dust (called loess). It happens that our property is at the very southern edge of this Iowa surface, and sand blew or was washed off the higher hills to our north, forming a large arc akin to an underground beach; the sand is under clay since in the last post-glacial 13,000 years, topsoil formed on top of the sand. Water runs off the Iowa surface and seeps out in low spots in this sandy band, so the pond below us is formed from damming a couple of these seeps.
So, I was right, after all, that sand was dumped on our land; I was just wrong about who did it, and I was off a little bit on the time scale...