Aspects of this topic are discussed in the following places at Britannica.
...bombardment and of the Moon’s thermal, particulate, and radiation environments. In the ancient past the stream of impacting bodies, some of which were quite large, turned over—or “gardened”—the lunar surface to a depth that is unknown but may have been as much as tens of kilometres. As the frequency of large impacts decreased, the gardening depth became shallower....
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