The journey upriver is a metaphor for the journey back to the beginnings of time. Marlow feels as though he is traveling through a prehistoric earth. This sense of timelessness is central to the novel's atmosphere. If you are looking to study this effect, a Heart of Darkness PDF is a great resource. It allows for quick reference to the specific passages where Conrad describes the overwhelming power of the jungle. The book remains a vital critique of the colonial mindset and a profound exploration of existential dread.