。
Thunderclouds hung over the entire valley.
Literal
Thunderclouds [subject-が] valley one-surface [over-に] hung-down.
たれこめる is a literary verb meaning '[clouds, mist, gloom] to hang heavily / blanket / settle low.' 一面に ('over the entire surface') is a fixed adverbial — 一面 literally 'one-surface,' meaning 'all over.' The whole sentence has an evocative, almost poetic feel.