Grammatically, the first sentence needs an article in front of "forest" . But it's often that writers and poets bend the rules, why is good : ) This writer probably wanted to make the rain forest feel like a dense substance like "walking through mud" We could imagine a similar sentence : "We fought our way through bush and thorn" "We flew our plane through cloud and mist" Makes sense?

The first sentence does not make sense. All nouns have to be preceded by "a", "an" or "the". If you want to express that there were multiple forests, then you would say "The trip took us through ancient, primordial rainforests."