。
Twilight was closing in on the valley.
Literal
Twilight [subject-が] valley [to-に] draw-near-[past-progressive-ていた].
A cinematic single-shot descriptive sentence. 迫る ('to draw near, press in, loom') carries a slightly ominous or weighty nuance, distinct from the neutral 近づく — twilight is closing in, not merely approaching. 谷間 ('valley, gorge') literally 'space between valleys/mountains' — the 間 suffix is 'space, interval, between.' Very characteristic of literary-descriptive prose.