Bullialdus has a high outer rim that is circular but observers have noted a slightly polygonal appearance.

The outer ramparts are covered in a wide ejecta blanket that highlights a radial pattern of low ridges and valleys.

When the Sun is at a high angle, the rim and central mountains appear brighter than the surroundings, and white patches can be viewed on the crater floor.

The impact may also have intersected a mafic pluton, which means a crystallized body of igneous rock that has high concentrations of heavier elements (such as magnesium, in this case).

[2] By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Bullialdus.