Geological Chronicle
North of Mare Serenitatis, in rugged lunar terrain, lies the heavily worn depression called Alexander. It sits south-southwest of the prominent crater Eudoxus and east-northeast of Calippus. The depression dates to the pre-Imbrian epoch, and erosion over time has left it looking like an enclosed lowland region surrounded by rugged ranges rather than an impact crater.
Surviving walls form a nearly rectangular outline along the northwest, west, and south sections of the depression, with the most prominent mounts rising in the northwest. The eastern side stands open to the surrounding surface, where the infrared spectrum of pure crystalline plagioclase has been identified on the east rim.
The floor dates to the Imbrian period and shows clear variations in tone and texture. Its western half is smoother and darker, while the surface gradually grows lighter and more impacted toward the east. A number of small hills rise across the floor, which contains no craters of significance despite plenty of tiny craterlets in the rougher eastern section.