亩

Top 亠 is the lid radical (Kangxi #8) - a short horizontal stroke with a dot above, an abstract cap that sits atop many chars (六, 京, 市). It is the indexing radical of 亩 by convention rather than by meaning; the lid simply marks where the field below is enclosed.

Bottom 田 is the field pictograph - a bird's-eye view of plots divided by paths, the iconic image of farmland in oracle bones. It carries the entire meaning: 亩 is a unit for measuring fields, the classical mu (about 1/15 hectare). The top lid is just a graphic cap; the field below is what the character is really about.