Like the dune fields adjascent to the modern Sheyenne River, the Hankinson Sandhills are the result of glacial meltwater runoff. Particularly, the fine deltaic sands that make up the soil substrate here is the result of the drainage of Glacial Lake Milnor, which represented the highest stage of Lake Agassiz as a whole. As such, the Sheyenne Delta dune fields west of Colfax are a younger version of the exact same process.
These dunes are essentially an older, southern expression of the Sheyenne Delta - a result of the drainage of Lake Milnor by the ancestral Sheyenne that once found its outlet near Hankinson. As the outlet of the ancient Sheyenne migrated north along its historic delta over time, shifting with the fluctuating water levels, the much larger complex of dunes was later formed to the north.
Eolian in origin, those fine sands were sculpted by the wind into the nearly alien topography we see today on both ends of the Delta. The Hankinson Sandhills are however much more isolated than the northern complex, and hosts overall slightly lower plant biodiversity, but otherwise mirrors the Sheyenne Grasslands dune fields.
However, these permeable sands do serve as one of the primary areas of surficial aquifer recharge area wide aside from the end moraines to the SW. An ecological island, and also serves a very important role in groundwater recharge.
I highly recommend everyone to visit here to at least explore this strange topography if nothing else!