Crater bottom structures show a consistent orientation in lunar cores. They have dust-free vesicular schlieren glass at the top, which grades downward to mafic arenite-a concentration of densely packed mafic mineral grains in a dark glass matrix. Next in the succession is a coarse-grained, polymict, compact and lineated breccia that has a nonporous brown glass matrix; it grades downward into a fine-grained, polymict lineated breccia that has a microporous glass matrix that is laden with fine fragments. All polymict regolith breccias formed by subcrater lithification show evidence of compaction (>2 contacts/grain in thin section), directional movement (linear fabric in at least some laced) and rapid cooling (glassy matrix but no reaction rims or obvious annealed grain margins). In contrast, suevitic polymict regolith breccias, which are lithified by heating, are not compacted (<2 contacts/grain in thin section, highly porous matrix), show evidence of rolling (radial-arcuate lapillar structures instead of linear fabric) and show sintered grain margins and/or reaction rims on mineral grains. The presence of relatively large fragments of shock-resistant minerals in the upper parts of the crater bottom structure, with other minerals being not clearly visible, suggests that fines and minerals easily damaged by shock are selectively fused to form the glass matrix.