Geological Chronicle
Bullialdus lies in the western part of Mare Nubium. Its relatively isolated location helps preserve its well-formed shape. The high outer rim is circular, although observers have noted a slightly polygonal appearance. Terraced inner walls show signs of landslips, and a broad ejecta blanket spreads outward in low ridges and valleys with a radial pattern.
Several peaks and rises in the center climb to over a kilometer in height. A raised ridge runs southeast from them before merging with the inner wall. The floor is generally rough, with many low rises, and its level portions are covered in lava younger than the impact. The floor has a somewhat convex shape, bulging upward toward the middle. At a high solar angle, the rim and central mountains appear brighter than their surroundings, while white patches can be seen on the floor.
Infrared studies of the crater region have revealed at least three layers of strata. The impact may have intersected a mafic pluton. Spectra from the orthopyroxene-rich central peak fit a gabbroic norite mineralogy, which originated from a depth of 6.0 to 9.2 km. Hydroxyl-bearing magmatic material has also been detected and was likely excavated from below the surface during the impact.