Small, fresh lunar craters with normal, central‐mound, flat‐bottomed, and concentric geometry are widespread on maria surfaces. The same types of craters have been produced in the laboratory by impacting projectiles against targets consisting of loose, granular, noncohesive materials overlying cohesive substrates. The mechanics of formation of each laboratory crater type is described, and evidence is offered that the corresponding types of lunar craters are of impact origin. Extensive studies of the effects of lunar impact variables on the conditions of formation of these crater types show that a previously described statistical method can be used to determine the thickness of the lunar surfaces layer within narrow limits. Two independent methods for determining the layer thickness at specific points are presented. Thickness estimates of the Surveyor 1 site obtained previously from study of medium‐resolution Orbiter 1 photographs are re‐evaluated by using subsequently obtained high‐resolution photographs, and thickness determinations of two additional areas are presented. The different areas examined have different surface layer thickness. The fragments of the surface are certainly partly of impact origin, but volcanic contributions may also be present. The maria substrates are probably composed of volcanic flow rocks with interbeds of fragmental material.

Many of the lunar surface formations have been emplaced by impact craters. Two mechanisms have been proposed for transport of crater ejecta; both the base surge and ballistic transport mechanisms are reviewed in this paper. Formation of base surges associated with underwater and underground explosion craters and with volcanic events all require the presence of an atmosphere in the area where ejecta impacts. Ejecta impacts and mixes with air and forms an aerosol cloud that carries the dust outward and deposits it on preexisting terrain. Because the moon contains no appreciable atmosphere, it is concluded that the base surge mechanism of separation and transport of fine‐grained crater ejecta is not a viable lunar process. Studies of laboratory impact craters, high‐explosion and nuclear craters, and lunar craters, and theoretical studies of formation of impact craters indicate that material is ejected from impact craters at low angles to the surface. Calculated ejecta positions at constant times after impact of the body that produced Copernicus crater are similar to shapes that are observed when laboratory craters are formed. Results are used to construct a model of emplacement of deposits of craters of all sizes. Particles are emplaced in low‐angle trajectories around the crater from the base of the expanding truncated cone. All of the ejecta of a small crater is emplaced at such a low velocity that the deposit consists entirely of crater ejecta. Therefore the deposit that surrounds the small lunar crater is entirely crater ejecta. When large craters are formed a significant fraction of the crater's ejecta has velocity high enough to crater preexisting terrain when it impacts. It craters preexisting terrain and mixes it with primary crater ejecta. The mixture of debris moves laterally away from the crater a short distance and forms the crater's deposit. It is concluded that deposits of large craters contain local preexisting material as well as crater ejecta. The cratering model is used to synthesize many lunar observations. For example, dunes around small lunar craters can be explained as a result of interaction of the flow with material ejected from secondary craters produced later at greater radial distances. Deceleration lobes and other features also are related to the emplacement model. Mantled Imbrium sculpture is explained as a result of production of the sculpture by secondary cratering due to passage of the conical sheet of ejecta and subsequent mantling of ejecta of secondary craters produced earlier nearer the crater. Model results, coupled with lunar observations, suggest that lunar smooth plains are in some places the erosional products of secondary craters of many highland craters and in some places they were emplaced by basins and consist of basin ejecta mixed with regional and local material. In some places they are predominantly deposits of local primary craters. The small low‐albedo smooth pondlike deposits that surround many lunar craters may be impact melts. If they are and if they were emplaced...

Laboratory impact cratering studies have been used to analyze the relationship between the crater size and crater morphology observed on the Lunar Orbiter 1 photographs. The results indicate that the fragmental surface layer is of variable thickness in the area of the Surveyor 1 landing site on Oceanus Procellarum. It is estimated that in 85% of the area the fragmental cover is between 5 and 15 meters thick. The estimated modal thickness is in the 5‐ to 6‐meter range, and the average thickness is from 8 to 9 meters. Differences are indicated in the average thickness between this region and other mare regions and between the maria and the highlands photographed by Orbiter 1. Such differences could have an age significance, implying that new rock surfaces on the moon have been formed at different times.

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