Let me break your 鬱蒼とした into the parts of the speech.
鬱蒼とした is separated into 鬱蒼 ( to Japanese language learners this is frequently called as na-adjective, but I would like to call it as quasi-adjective ( adjective noun ), and here, it is used as the stem of the quasi-adjective = A noun that functions as an adjective. ( Kindly be reminded there are many disputes regarding how to deal with this as the na-adjective or if it is the noun+auxiliary etc. ( For example, Motoki Tokieda is taking the latter stance. Here I would like to the latter stance. ) meaning, profuse, dense, thick etc, ) / と, case-marking particle, followed by the substantive, ( please see grammar ), meaning like, as if, etc ( Please refer to the further information below ) / し ( 連用形（れんようけい）,the continuative form ( meaning, to be continued to such as verbs, auxiliaries etc.. ) of the verb する, meaning, do or be ( this case ) / た, 連体形,the attributive form ( meaning, after this follows a noun, so in your sentence the noun 林, forest ) of the auxiliary た, which denotes here the continuation. )
So if you take not 鬱蒼とした, but 鬱蒼とした林, after the long and complicated Japanese grammar above, it would stand as a subjective, meaning "dense, thick, dark, etc, forest".
Now let's take a look at the case marking particle
From the source, 4,
meaning, 乾草, a noun, meaning dried grasses or hay,
The translation will be
the accumulated hay like a mountain.
Doesn't it sound similar?
Hope I could somehow help.
Have a happy day.