taru-adjective, adverb taking the 'to' particle
Literary or formal adverb/adjective describing something that stretches on without interruption, often used for landscapes, time, or abstract continuity. Typically appears as 綿々と or 綿々たる.
山脈が綿々と続いている。
The mountain range stretches on unbroken.
彼の話は綿々として終わらなかった。
His story went on endlessly without finishing.
延々 also means 'endlessly' or 'on and on', but is more common in everyday speech and often carries a negative nuance of being tediously long. 綿々 is more literary and neutral, emphasizing unbroken continuity.
Reduplication of 綿 (めん, 'cotton; continuous'), emphasizing the idea of something stretching out like a long thread. The exact historical derivation is uncertain, but the spelling is conventionally associated with unbroken continuity.