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.

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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