Stratigraphy
The rock units are subdivided into three major groups: plains materials, terra materials, and crater and basin materials. The plains and smooth terra units are considered to be volcanic in part, and thus may have a different origin from the impact breccias and churned regolith forming the rough terra and crater deposits.
The oldest rocks exposed in the quadrangle are the intercrater plains material and the rims of the oldest craters and basins. Collectively, these rocks form a relatively subdued terrain of moderate relief. They are similar to some of the rolling and hilly terra and hilly and pitted materials in the southern lunar highlands, particularly in the Purbach (Holt, 1974) and Tycho (Pohn, 1972) quadrangles. The intercrater plains unit is commonly marked by the soft outlines of numerous overlapping secondary craters producing a subdued hummocky texture. It is gradational in places with cratered plains material, which forms flat, densely cratered surfaces similar to pre-Imbrian plains on the Moon (Wilhelms andMcCauley, 1971; Scott, 1972) Although both the cratered and intercrater plains deposits are interpreted to be volcanic, the latter has been highly degraded by repeated impacts over a longer period of time. Much of its surface is probably covered by a relatively thick regolith of reworked impact breccias.
The cratered plains material is relatively flat with broad ridges and lobate scarps that in places resemble those of some of the lunar maria. It is difficult to obtain reliable crater counts on this unit because many secondary craters cannot be distinguished from primary craters. Cratered plains materials embay craters in classes c1 to c3; they may represent lava flows extruded after an initial phase ofimpact flux. The albedo of the cratered plains is intermediate compared to that of other mercurian units, but higher than that of the lunar maria, and may reflect lower iron and titanium content.
The youngest rock units consist of rough terra and smooth plains materials. Rough terra occurs as overlapping and intermixed ejecta blankets around dusters of large young craters in the eastern part of the quadrangle. The relief here appears tobe higher than elsewhere in the map area, and the occurrence of dense arrays of fresh secondary craters produces a coarsely textured, hummocky surface at a scaleof about 10–20 km. The effect of roughness is highlighted by the low sun illumination angle. Ordinarily, rough terra material would be subdivided and mapped as individual ejecta blankets around and belonging to particular craters. In this eastern region, however, the closely grouped craters have about the same age, and it has not been possible to distinguish the boundaries between their aprons in many places.
Smooth plains material covers the floors of numerous craters in all age classifications. Its surface is scoured by secondary craters from classes c4 and c5 craters at many places in the eastern part of the quadrangle and, within the crater Homer (1° S., 37° ), by secondaries from the class c3 craters Titian (3° S., 42° ) and Handel (4° N., 34° ). Thus the smooth plains unit may have a relatively wide agerange. Like the cratered plains, it exhibits lobate scarps and few mare-like ridges, but these are generally smaller than those of the cratered plains and more nearly resemble those of the lunar maria. Although crater counts are more reliable because there are fewer secondaries than in the cratered plains, resolution is a serious constraint to developing crater counts on the relatively small tracts of smooth plains. Preliminary counts made on a few of the more extensive occurrences of smooth plains show a cumulative crater frequency of about 7.5 × 102/106 km2 for craters larger than approximately 2.5 km. This frequency is comparable to that of the lunar maria near the Apollo 11 landing site (Greeley and Gault, 1970; Neukum andothers, 1975; Meyer and Grolier, 1977). Like that of the cratered plains, the albedo of the smooth plains is intermediate compared to other units on Mercury but is high compared to that of the mare basalt on the Moon.
A few patches of very dark material occur in the western part of the quadrangle where the sun angle is high and albedo contrasts are enhanced. The largest of these dark patches is apparently superposed on the bright rays of a c5 crater and is therefore very young.
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