SPOILER ALERT: This question asks about the last line of Heart of Darkness by Joseph Conrad. If you are reading the novel, you may want to skip this question.
The offing was barred by a black bank of clouds, and the tranquil waterway leading to the uttermost ends of the earth flowed sombre under an overcast sky – seemed to lead into the heart of an immense darkness.
Should an adverb (i.e. sombrely) have been used to describe 'flowed'?
Are there any formal terms describing this issue? I question my doubt given Conrad's renown.
Obiter dictum: I lighted upon this while reading The Guardian's 'The 10 best… closing lines of books'.