The Caloris Montes Formation is very similar in morphology to and is considered the equivalent of the massif facies of the Montes Rook Formation around the Orientale Basin. The Caloris Montes is interpreted as basin rim deposits consisting of ejecta from deep within Caloris that is mixed with but generally overlies uplifted and highly fractured prebasin bedrock. A gap is present in the Caloris Montes toward the southeast; its origin is unknown, but it is somewhat similar to the gap on the east side of the Imbrium Basin, where the mountain ring cuts the edge of the Serenitatis Basin. On Mercury, however, we have no evidence for the presence of a preexisting basin east of Caloris. Nervo Formation The Nervo Formation consists of rolling to locally hummocky plains that lie in intermassif depressions between the mountains formed by the Caloris Montes Formation. The plains generally lie within the annulus of rugged terrain marked by the Caloris Montes Formation and locally appear to drape and overlie some of the more low-lying massifs. The Nervo bears some resemblance to the Apennine Bench Formation around the Imbrium Basin; its closest counterpart in Orientale is the knobby facies of the Montes Rook Formation. The Nervo Formation was originally designated the intermontane plains by Trask and Guest and has been interpreted by them as fallback ejecta, an interpretation that seems to explain its distribution pattern and relative roughness as well as the fact that it is generally perched above the smooth plains that encompass Caloris. The Nervo formation is named after Nervo crater. Odin Formation The Odin Formation, which was originally called the hummocky plains by Trask and Guest, was described by them as consisting of low, closely spaced to scattered smooth hills about 0.3 to 1 km across and from tens of meters to a few hundred meters high. In some places the hills are aligned concentrically with the rim to the Caloris Basin, and the plains appear corrugated. The area between the hills is similar in appearance to the smooth plains; in some areas the Odin Formation may be partly flooded by smooth plains materials, but for mapping convenience this area has been included in the Odin Formation. Because the relief on the Odin Formation is low, identification depends much on image resolution and lighting, and some outcrops may not have been recognized. The distribution pattern of the Odin appears similar to that