The aim of this paper is to point out that if the sinuous rilles on the Moon represent trenches in the mare ground in which they meander, the existence of a great number of individual boulders on their slopes - as discovered on the high-resolution photographs taken by US Lunar Orbiters 4 and 5 in 1967 - suggests that the solid substrate of the lunar globe is covered by broken-up debris produced by cosmic abrasion - and hereafter referred to as lunar regolith - of thickness comparable with the depth of the respective rilles - at least of those lacking flat floors; which is generally in the order of 200–300 m. This depth is much greater than that indicated previously by other methods possessing more limited depth in range; and need not apply uniformly all over the Moon. In point of fact, marial regions abounding in sinuous rilles may represent loci where the lunar regolith has developed its maximum depth.