I transcribed some of the collar data from the nearside plate of the USGS Geologic Lunar Atlas to get a general background on what we see.
GEOLOGIC SUMMARY
MARIA AND MULTI-RING BASINS
The circular basins, generally encompassed by three or more partly complete scarps or ring systems, dominate the geology of the Moon’s near side. Diameters of the innermost ring range from 250 to 600 km (kilometers), and the spacing of the three rings increases from one to the next by a regular ratio (Hartmann and Kuiper, 1962). The observed and inferred crest lines of the most important of these structures are shown by brown lines on the accompanying map. The depressed central parts of the basins and the concentric troughs which lie between successive rings are completely or partially filled with dark plains material (mare) which is younger than the basins and which appears to have been emplaced in a fluid and probably molten state. These basins and their subsequent mare fill have destroyed large parts of the earlier geologic record, and in this respect have acted like terrestrial orogenies. The historical geology of the Moon can be understood only in terms of these dominant, primitive structures which, if ever present on Earth, have long since been obliterated or deeply buried and severely modified.
The general circularity of the basins, the textural similarity of the materials of the ring structures, and the regularity of the ring spacing suggest a common origin. If all basin-related features are considered, particularly for the best preserved basins, an impact origin, as first suggested by Gilbert (1893), is strongly favored. The inner rugged rings of the two best preserved basins, Orientale (just outside the map area) and Imbrium, are surrounded by distinctive ejecta blankets (Hevelius Formation and Fra Mauro Formation respectively) which can be traced outward for hundreds of kilometers. Relicts of what is probably a third, more degraded and older ejecta blanket (Janssen Formation) can be recognized south of the Nectaris basin. Within and beyond these blankets, chains and clusters of craters have been identified as probable secondary impact craters on the basis of morphology, size, and distance from basin (see section on crater materials). Observable differences among basins -- such as the presence or absence of a recognizable blanket, the completeness of the concentric ring structures, the freshness and relief of the rings, the character of the radial structures, and the type and numbers of superposed craters -- can be explained by variations in relative age, degree of filling by younger mare, or proximity to younger basins. Alternative explanations for the multi-ring basins calling for an internal origin (Spurr, 1944; Green, in Oriti and Green, 1967) are inadequate because of failure to account for all of the basins’ observable properties, relative ages, and relations to subjacent and superposed units such as the maria.
A considerable body of stratigraphic evidence shows that mare material in the basins is not the immediate and direct product of the basin-forming impact but is a later volcanic fill: (1) The mare truncates all other basin-related deposits and structures. (2) It is located in basin-related topographic lows, including troughs between the outer rings far from the basin centers. (3) The mare surface materials in all basins appear to have a narrower age range (determined from the number of superposed craters) than the basins, which differ greatly in state of preservation and apparent relative age. (4) Craters are superposed on the basin ejecta or basin-related structures but are embayed or filled by mare material occur in every basin (for example, Posidonius, Gassendi, Cleomedes, and Fracastorius in the Serenitatis, Humorum, Crisium, and Nectaris basing respectively). The postbasin, premare craters within the Imbrium basin (that is, Imbrian-age craters) are approximately equal in number to the total number of postmare craters (Eratosthenian and Copernican ages) in the same area. Thus a considerable interval between basin formation and final mare filling is suggested. (5) A distinct geologic unit, light plains material (Ip), lies stratigraphically between the deposits of the Imbrium basin and postbasin, premare crater materials, including those of Archimedes, Cassini, and Plato. (6) Features far from and unrelated to the Imbrium basin are also of Imbrian age, demonstrating an extensive history between the formation of the basin and the last mare fill. Orientale is one such Imbrian-age feature, and many Imbrian craters apart from those in the Imbrium basin both predate and postdate Orientale. (7) The nearside basins are filled by mare to varying structural levels independently of their size. For example, Nectaris is filled to the first ring and Crisium and Imbrium to the second and third respectively. Uneven filling suggests different volumes of mare material in each, reflecting internal rather than external (impact melting) controls on their filling.
Each basin immediately after excavation and early gravitational slumping probably resembled the present youthful-appearing Orientale basin (McVauley, 1967b, -. 439-446; 1968). Orientale is the key to the internal configuration of the more degraded and deeply filled basins because of its lack of extensive mare filling and its relative youth. Its present shallow profile suggests that the thickness of the mare fill even in more deeply flooded and obscured basins like Imbrium is on the order of only 2 to 5 km. Mare material, although covering almost 30 percent of the nearside surface (and very little of the far side) is, therefore, volumetrically a small part of the Moon (radius 1738 km). The deeply flooded basins like Imbrium may lie in regions where potential source material was at shallower depth than it was in the less filled basins like Orientale and Nectaris. The existence of numerous thalassoids (basins without mare fill) on the far side of the Moon (Lipinskit, 1965) further supports the conclusion that the location of a basin and local crustal conditions determine the degree of later flooding.
Oceanus Procellarum is unusual because of its non-circular, somewhat irregular shape. It is the largest expanse of mare on the Moon that is not obviously controlled by a single circular multi-ringed basin. It contains throughout most of its extent, numerous terra islands and large ghost or vestigial craters such as the Flamsteed ring which collectively suggests that the mare is relatively shallow. Its overall shape however, appears to be controlled by outer ring structure of the Imbrium basin and two vague, ancient ring structures centered near Copernicus and Flamsetted which were identified during this work.
During and after the period of flooding, meteorite bombardment eroded and subdued the basin rings and blankest through direct hits, impact of far-flung secondary ballistic fragments (Shoemaker, 1965), and seismic shaking of the surrounding terrain triggering downslope movement of loose materials (Titley, 1966). This process led to the eventual disappearance of the distinctive depositional textures of the circumbasin blankets. Catastrophic premature aging and partial obliteration of basins is exemplified by Serenitatis, whose rings have been disturbed structurally and mantled by ejecta from the nearby Imbrium basin. The trend of the younger Imbrium rings has in turn been influenced by the presence of the earlier Serenitatis structure as seen in the change in the trend of the “Apennine” ring in the Montes Caucasus region.
TERRA PLAIN, PLATEAU, AND DOME MATERIALS
Prior to formation of the youngest mare material, the basins were also modified by the deposition of several types of distinctive terra units described in detail in the accompanying explanation. The most extensive of these are the light terra plains materials of Imbrian age (unit Ip) which lie predominantly in circumscribed throughs and on the floors of pre-Imbrian and lower Imbrian craters. Plains identified as pre-Imbrian (pIp) by superposition of lowermost Imbrian crater clusters occupy more restricted areas in the southern terra. These two marelike plains units may represent earlier generations of volcanism and crustal flooding, their present high albedo relative to the mare material being due either to initial compositional differences or to more extensive churning and consequent brightening by prolonged meteorite bombardment. The most distinctive terra unit (hilly and furrowed material, Ihf) forms level or shieldlike plateaus that cover basin ejecta and postbasin craters and contain numerous small primary volcanic landforms. The two largest occurrences lie west of Nectaris between the third and fourth rings and narrowest of Humorum athwart and beyond the third ring. These plain and plateau units, which apparently modify earlier basin or crater deposits, suggest a more extensive history of lunar volcanism than previously recognized. Moreover, they differ markedly in albedo and roughness from the dark, relatively smooth mare volcanic deposits and they may thus represent the products of a distinct terra magma or of mare magma contaminated by terra materials. The bulk differences in composition from the maria could, however, be minor, as in the case of terrestrial cone and flow materials within the same volcanic field. The distribution of the terra volcanic units is controlled predominantly by basin ring structures; some occur at the intersection of younger rings structures with vague circular depressions which are the probable relicts of very ancient basins. In addition to these more extensive deposits, several types of domes, mostly in the 10 km size range, have been identified both in the maria (units Emd, Emp) and in the terrae (units Id, CEd). Many of these, including the largest and most distinctive, the postmare terra domes of the Gruithuisen area southwest of Sinus Iridum, lie along circumbasin ring structures.
CRATER MATERIALS
Craters of all ages, mostly circular, dominate the lunar landscape at all scales. The circular craters, except those deeply buried or in a poor state of preservation because of their antiquity, have physiographic characteristics that indicate an explosive origin (Baldwin, 1949, 1963). Their cumulative frequency distributions are difficult to explain by a volcanic model (Green, 1962) but do fit well with an impact model (Baldwin, 1963). The circular craters, probably of primarily impact origin, are here grouped in a “main sequence” divided into eight age categories.
Foremost among the craters not grouped in the “main sequence” are chains and clusters of 10-15 km bowl-shaped or irregular craters lying within and beyond the confines of the Hevelius Formation (out to 1500 km from the Orientale basin center) and confidently identifies as secondary impact craters formed by ejecta from the Orientale basin unit Isc). Similar but larger, less well preserved clusters and chains of clearly Imbrian craters can be identified on the pre-Imbrian terrae as probable secondary craters of the Imbrium basin (unit Icc). Many degraded or partly buried pre-Imbrian chains and clusters (unit pIch) near the edge of the Janssen Formation may be secondary craters of the moderately well preserved Nectaris basin. Thus secondary craters of large basins constitute an important part of the total lunar crater population in the 10-25 km size range on the older parts of the terrae. A few smooth, low-rimmed craters such as Lassell and Damoiseau, mostly in the 20 to 30 km range, lack impact characteristics and could be calderas (unit Ics). Certain irregular craters and clusters and chains of smooth-rimmed craters that are controlled by local or regional structural trends are also considered to be volcanic features. Most of these are relatively small and relatively young (units Ich, Ici, CEch, CEci), but some large irregular craters (pIci) and chains (pIch) of probable pre-Imbrian age could also be volcanic, suggesting a possible early epoch of crater-forming volcanism.
A number of craters, particularly those of the older subdued variety such as Pitatus, exhibit numerous superposed volcanic landforms such as domes and crater chains (mostly too small to be mapped as separate features). The presence of these landforms suggests either that the host craters are volcanic or that impact scars may be the loci of long-subsequent volcanism even for features much smaller than basins.
GEOLOGIC HISTORY
From these regional observations and interpretations a geological history of the Moon can be extensively defined. The relative ages of the multi-ring basins can be partly established by overlap relations with one another, relative topographic freshness, and age of superposed and subajacent craters. Orientale is younger than Imbrium; Nectaris, Crisium, and Humorum are older bt difficult to date with respect to one another because of their geographic separation; Serenitatis and Fecunditatis are next in sequence, and Nubium and Tranquillitatis are the most ancient of the prominent multi-ring basins. Each basin is inferred to have undergone its own cycle of early volcanic filling, impact erosion, and mantling by thick or thin to discontinuous ejecta from nearby and distant younger basins, and each contains mare fill in various amounts. Thus the stratigraphic sections in and around each of these basins are complex and reflect variable local interplay between external and internal lunar processes.
The most primitive appearing terrain on the near side lies in the south central part of the map area extending as a wedge-shaped tract broadening from Ptolemaeus to the region of the south pole. This province consists predominantly of closely packed 100-200 km pre-Imbrian craters of the probable impact sequence along with chains and clusters of Imbrian and probably Nectaris secondary craters. The preservation of this topographically positive area, within which volcanic filling played a role of uncertain importance, is explained by its location beyond the range of extensive blanketing and structural disturbance of the recognizable circular basins.
The regions south and southeast of Mare Humorum, between Serenitatis and Crisium, and southeast of Crisium also consist of basically pre-Imbrian terrain and represent the next most primitive part of the Moon’s near side. This essentially nondescript terrain, characterized by a moderate deficiency of large pre-Imbrian craters, is considered to represent an interlayered sequence of now unrecognizable crater material, volcanic material, and basin ejecta blankets; each blanket originally resembled that around Orientale and buried much of the earlier local geological record.
The lunar stratigraphic record becomes clearer with the catastrophic formation of the Imbrium basin which began the Imbrian Period. The basin and the blanket, however, have undergone considerable modification of original textures is attributed to subsequent meteorite bombardment, but some is also due to volcanism. Extensive light plains materials filled low areas within the basin and in the troughs between rings, truncating the braided and hummocky textures of the blanket and basin radial structures. Dark blanketing material was deposited in a broad arc midway between the third and fourth Imbrium rings. Steep domes and other distinctive terra landforms were also formed. These various blanketing and filling materials range from early Imbrian to Copernican in age, further supporting the conclusion that basin modification by volcanic processes is of long duration.
Some time after formation of the Imbrium basin, extensive modification of parts of the older terrae began, especially northwest of Mare Humorum and west of Mare Nectaris. Distinctive terra landforms were formed near these older basins in greater number than near Imbrium, a fact which may reflect time dependence in their formative process, possibly magmatic differentiation as the Moon evolved.
The most widespread flooding of the depressed parts of the lunar crust occurred near the end of the Imbrian Period with the filling of most of the nearside multi-ring basins by mare material. This was followed by a less extensive episode of mare generation, terra blanketing, and volcanic mare plateau formation (as in Marius Hills), mostly in the Eratosthenian Period.
Impact cratering, at a rate probably substantially reduced from that of pre-Imbrian to early Imbrian time, continued simultaneously with these events. The most recent impacts produced the prominent ray craters such as Copernicus and Tycho, whose ejecta blankets are superposed on almost all other units. No widespread volcanic activity appears to have occurred since this cratering, although some activity, as in the Littrow region at the southeastern edge of Mare Serenitiatis, may be approximately contemporaneous with it.
GENERAL CONCLUSION
Systematic regional mapping shows the Moon to be a primitive body without the orogenic belts, mobile plates, oceanic ridges, and widespread waterlaid sedimentary deposits characteristic of the Earth’s crust. The Moon does, however, exhibit a geologically heterogenous surface with a long complex history which can be partly unraveled from existing photographic data. Geologic mapping reveals a Moon that is neither dominantly “volcanic” nor dominantly “impact” but rather one in which both processes have been operative. Most of the volcanic units are restricted to regionally depressed areas, the distribution of which is controlled by ancient multi-ring basins formed by impact relatively early in lunar history. Implicit in this historical model is that a moderate amount of magmatic fractionation and vertical differentiation took place throughout at least the middle parts of lunar history (Imbrian Period). Thus most of the present surface, although less reworked than that of the Earth, is neither primordial nor of meteoritic composition. However, even the mare rocks, which are relatively young on the lunar timescale, have been shown by preliminary results from Apollos 11 and 12 to be as old or older than any presently dated terrestrial rocks. Thus the geologic record of the Moon appears to complement that of the Earth by covering the period of time for which the record is missing on Earth.
