taru-adjective, adverb taking the 'to' particle
Literary taru-adjective describing a bluish, dim, or dusky appearance, often of vegetation, sky, or twilight. Rare in modern Japanese; mostly found in classical or poetic contexts.
夕暮れの森は蒼然としていた。
The forest at dusk was dim and bluish.
蒼然たる山々が遠くに連なっている。
Bluish mountains stretch out in the distance.
The kanji 蒼 means 'blue' or 'green' and 然 is a suffix forming adjectives/adverbs. The exact historical derivation is uncertain; the word is a classical Chinese-style compound.