As nouns the difference between marl and mudstone

is that marl is a mixed earthy substance, consisting of carbonate of lime, clay, and possibly sand, in very variable proportions, and accordingly designated as calcareous, clayey, or sandy while mudstone is a fine-grained sedimentary rock whose original constituents were clays or muds.

As a verb marl

is to cover, as part of a rope, with marline, marking a peculiar hitch at each turn to prevent unwinding.