Geological Chronicle
Palus Somni, translated from Latin as the Marsh of Sleep, is an area of relatively level but somewhat uneven terrain along the northeastern edge of Mare Tranquillitatis and Sinus Concordiae. Spanning a diameter of 163.45 kilometers, its surface has low ridges and patches of level terrain. It features a higher albedo than the lunar mare to the west, displaying a shade of grey typical of continental terrain.
A few minor craters lie within its borders, with the flooded Lyell along the west edge, Crile to the east, and Franz to the northwest, while the bright crater Proclus lies to the northeast. In 1907, it was described as having a unique light brown color, quite unlike the hue of any of the other plains or mountain regions.