Up and Down the Atlantic coast line are oval and elliptical depressions that are usually called Carolina Bays or Delaware Bays or whale wallows. In Delaware they are mostly found in the Eastern side of Delaware, but some are scatter about in the western half. No one knows how they were created so speculation goes from sinkholes, to meteor impact, to ancient whales wallowing in the sand as the water receded leaving them stranded. Extra-terrestrial landing pads is my theory. They frequently are oriented to a NW - SE axis. Anyway, to the West of Delmar on Old Racetrack road is one. It is the yellow circle labeled QCD. You can't see it from the road due to trees around it but it is shown on the Delaware Geological Survey Map Number 22, for which there is a link below
If you actually go to the link you will see there is another one out by the corner of the Maryland Delaware state line. There are also a number of ancient sand dunes shown on the map.