。
Leaves lay thick all over the ground.
Literal
Tree-[possessive-の]-leaves [subject-が] ground [one-surface-一面] [on-に] thickly was-piling-up.
一面に ('all over, across the whole surface') is a fixed adverbial phrase — 一 ('one') + 面 ('surface') — used to convey that something covers an entire area. 積もる ('to pile up, to accumulate') in ~ていた marks the resulting state of accumulation at a past reference point: the leaves had already built up by the moment being described.