The compound adjective 'well-marked' is a transparent combination of two Germanic elements. The adverb 'well' derives from the Proto-Germanic wel, carrying the sense of 'pleasingly' or 'thoroughly,' which evolved into its modern intensifying function. The adjective 'marked' originates from the Old English verb mearcian, meaning 'to trace a boundary' or 'to designate,' a concept rooted in the practical need to identify territory or objects. When combined, the literal sense of 'thoroughly designated' logically extends to the modern abstract meaning of something being clearly defined, evident, or distinct, such as a well-marked trail or a well-marked difference.

Structure: well (thoroughly) + marked (indicated) - well: From Old English wel (in a good manner, thoroughly). In this compound, it functions as an intensifying adverb modifying the adjective. - marked: From Old English mearcian (to mark). In this compound, it functions as the core adjective meaning 'indicated' or 'having a mark'.