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The Geminus quadrangle, centrally located in the northeast quadrant of the lunar near side, is dominated geologically by several multi-ringed circular basins outside the quadrangle and five relatively young, large craters mapped here. The terra and the older craters apparently are mantled by a nearly continuous blanket of ejecta from the Serenitatis, Crisium, Humboldtianum, and Imbrium basins, which lie, respectively, southwest, southeast, and west of the quadrangle. Escarpments and rings of blocks concentric with these basins were raised by faulting and tilting when the basins were formed, presumably by hyper-velocity impacts of asteroid-sized bodies. The light and dark plains in Lacus Somniorum and in the north...
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The Syrtis Major quadrangle is located in the equatorial belt of Mars across the boundary between the cratered plateau region to the south and west and the northern lowland plains. The cratered plateau is part of the northern extension of cratered highlands, which generally stand at altitudes greater than 4 km. The southeastern part of the quadrangle is dominated by eastward-sloping plains on the floors of Sytris Major and Isidis Planitia. Total relief in the quadrangle is greater than 6 km over a latitudinal distance of 2,000 km, corresponding to a regional slope of 0.6 degree in an east-northeast direction.
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Most of the photographs used for geologic mapping were acquired by the departing spacecraft during the first pass (Mercury I). The Mercury II encounter provided no usable images of the map area; two low-oblique photographs suitable for geologic mapping were acquired during the third flyby on March 17, 1975 (Davies and others, 1978, p. 31). No stereoscopic phtographic pairs are available for the Borealis region.
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