Caloris Basin
The Caloris Basin is especially significant from a stratigraphic standpoint. Like the Imbrium and Orientale Basins on the Moon, it is surrounded by an extensive and well-preserved ejecta blanket. As on the Moon, where ejecta from the better preserved basins was used to construct a stratigraphy, the ejecta from the Caloris Basin also can be used as a marker horizon. This ejecta is recognizable to a distance of about one basin diameter in the Tolstoj quadrangle and the adjacent Shakespeare quadrangle to the north. Undoubtedly, the ejecta also influences a large part of the as-yet-unseen terrain to the west. A stratigraphic and structural comparison between the Orientale and Caloris Basins has been made by McCauley. McCauley and others have proposed a formal rock stratigraphy for the Caloris Basin that we have adopted on the present map. This stratigraphy is patterned after that used in and around the Orientale Basin on the Moon and should aid in the future recognition of pre- and post-Caloris events over a broad expanse of the surface of Mercury. Crater degradation chronologies, such as the one modified from Trask, and correlations between plains units on the basis of crater frequency may aid in tying much of the remainder of the surface of Mercury to the Caloris event.