Tadami Town is in the Minami-Aizu region of Fukushima Prefecture, situated on the border with Niigata, separated by steep mountains.

Literal

Tadami-Town [topic-は] Fukushima-Prefecture Minami-Aizu region [in-に] exist, steep mountains [object-を] separating Niigata [with-との] prefectural-border [at-に] positioned.

A geographic description. 急峻な山 'steep mountains.' を隔てて 'separating, with X in between' — 隔てる 'to separate.' 県境 'prefectural border.' The sentence reads like a travel guide or encyclopedia entry.