The lunar maria show some very distinctive characteristics:
1. They are much darker in appearance than the surrounding rock.
2. They tend to be very wide spread, but very flat and smooth.
3. They tend to be bounded by ridges, walls and craters (i.e. higher altitude areas).
These characteristics point to their composition and formation: The dark colour comes from basalt, which indicates that the rock is volcanic in origin. The smoothness of the maria suggests they were formed by molten lava, rather than by erosion or impact. And their relative low altitude adds to the evidence that they were formed by molten rock flowing into the lower-level areas of the lunar surface.