Montes Rook is a ring-shaped mountain range that lies along the western limb of the Moon, crossing over to the far side.

It completely encircles the Mare Orientale, and forms part of a massive impact basin feature.

The material excavated to form Montes Rook came from a mafic layer below an anorthositic zone.

[1] Sections of the gap between these sub-ranges contain long valleys filled in places with basaltic lava, forming small lunar maria.

This is how William K. Hartmann and Gerard Kuiper discovered the ring shape of Montes Rook in the early 1960s.