A petrographic study of eleven samples of clastic rock returned from the moon by Apollo 15 suggests that two lithologies are present. The distinction between the two lithologies is based on the glass content of the rock matrices and the morphology of the detrital particles. Group I rocks have abundant, glass-rich, porous matrices and glass particles with morphologies comparable to those of glass particles in the lunar soil. The group I rocks were probably formed by welding or sintering of surficial soil deposits by impact-generated base surges of limited extent. The base surge clouds contained a relatively low-volume concentration of particulate material. Particle morphology was thus determined largely by the major soil-forming processes, namely comminution and vitrification by meteorite impact. Relative abundances of lithic clasts in the rocks suggests that all were formed locally.

Group II rocks have an essentially mineralic matrix and have an abundance of rounded mineral grains. Sample 15455 is the only Apollo 15 sample assigned to this group. In its general textural features, sample 15455 is comparable with the group II rocks from the Fra Mauro Formation at the Apollo 14 site. Textural features such as shock modification and rounding of mineral grains suggest that this sample is the product of a large-scale impact-generated base surge which possibly resulted from the Imbrian event. In contrast to the group I rocks, the group II rocks were probably deposited from a base surge with a high-volume concentration of solids. Consequently, particle morphology was determined largely by abrasion in the base surge.

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