Caloris Group

The Caloris Montes Formation, which was informally called the Caloris mountains terrain by Trask and Guest, consists of a jumbled array of smooth-appearing but highly segmented mountain massifs that rise 1-2 km above the surrounding terrain. These massifs mark the crestline of the most prominent scarp or ring of the Caloris Basin and grade outward into smaller blocks and lineated terrain. 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, and Denevi et al. The distribution pattern of the Odin appears similar to that of the thinner, more distal parts of Alpes Formation of the Imbrium Basin on the Moon. The Odin, like the Alpes, occurs in broad lobes such as those in Odin Planitia beyond the main basin scarp. The Odin also mantles the intercrater plains ancient crater materials out to a distance of 1200 km from the main Caloris scarp. The Odin Formation is interpreted as part of the Caloris ejecta sequence, but its mode of origin is less clear than those of certain other Caloris formations. The unit may consist of late-arriving, blocky, coherent ejecta from deep within the Caloris cavity, later partly buried by smooth plains. Odin locally mantles intercrater plains, lineated plains, and intermediate plains materials to a distance of 1,100 km from the Caloris Basin scarp. It is named after Odin Planitia, its "type area". It is also mapped in Mearcair Planitia, Stilbon Planitia, and Tir Planitia. ==Van Eyck Formation==

The Van Eyck Formation, which is the most distinctive of the circum-Caloris stratigraphic units, was called the Caloris lineated terrain by Trask and Guest. Over much of its outcrop, it appears to be veneered by a thin plains unit that has filled hollows in the surface. The plains unit in these areas has generally been infilled with the Van Eyck Formation, although it may, in part, be smooth plains material. ==References==