。
The field was covered in deep snow.
Literal
Field [topic-は] deep snow [in-に] was-covered-[ていた].
Particles に versus で give different shades for what's covering the field: に here marks 'in/by (deep snow),' emphasizing the state of being enveloped, while で elsewhere marks the snow as instrument or material. Both are grammatical with 覆われる; the choice is a matter of nuance. 深い modifies 雪 as a prenominal adjective.