Gridrock

Where dunes and grass meet, the ground grows thick at the crust; porous, hard as the hilt of a sword, with sudden drops and occasional sand that slithers in winds across it.
Much of that sand falls down a visible gridwork of rusted steel down between forged gaps, looking very much like the sieve of a giant gold-panner being drawn from sparse prairie straight into the dunes.

Deeper: Tufts of bent grass on the dunes blend into a caliche being overcome by grassland growth. Something has been wetting the area so the soil beside the hard build-up is soft in the way that lets things grow.
Something sank here, an ancient seaside station, but so many years ago now that the buildings beneath have knelt to the pressure of burial and only rise up as rusted corten panels and components useful for their metal and little else. And even those have been gnawed at, often.
The ground is hard, but a strange honeycomb grid on the backside of the caliche gives it its name. Gridrock. An ancient sewer tunnel exit. A great iron portcullis is half buried in the natural concrete and half exposed among the grasses, like a massive 200' coin cemented at the border of a sidewalk and a yard.
The gaps in the grid are angular—yawning steel-sided gaps three and a half feet thick, rusted to hell. Some things were revealed with the departure of the sea, other things were hidden.
When sand blows on it in from the deader lands, it flows down where the gaps creating a sloping hill under the grate that leads down into the tunnel the steel once filtered—the seaside station's secondary entrance.
A happy coagulate of teurk slime gather in the tunnel and exchange base sediments to freshen themselves. They are, largely, the reason for the ecological hybridity of the space: trees grow where the teurk slime go. 
Should excavation be made of a serious effort, the great tunnel at base would be discovered the exterior to Perdock Site Naval Base detailed on P.XX, a place as undoubtedly alien to modern minds as anything from distant galaxies.
It was once a manned station by a breakaway sect from the ring-ship now known as Nephew. The civilians within lasted long enough to build a colony of fifty-five. Again, this would take the kind of excavation done by a hundred men over four or five years. Or someone with a dedicated burst of creativity.