So I am writing a letter in past tense about a recent camping

>>17368857
English teacher here.

Either tense will work. The difference is a small one of nuance.

If you say "were" you are staying inside the story - this happened and that happened and at that moment I was impressed by the mountains

If you say "are" you are briefly breaking the frame - this happened and that happened and, stepping outside the story for a moment, I still think those are beautiful mountains. Now back to my story....

The same is true of is/was.

If you understand the small distinction I'm making, decide which you want to say. If you don't understand then do whichever you want - the difference is tiny.

>>17368905
This 2bh. Either tense makes sense in both cases