Break the logs in the Kid's path to encounter a friendly Gasfella. A gate blocks the path forward, so take the side path to a switch that opens it. Past the newly-opened gate is another gate that must be opened with switch, along with some Swampweed plants. These plants are so weak that simply coming into contact with them will destroy them, but doing so releases a poisonous gas that lingers in the air. This gas not only hurts the Kid, but also distorts his vision, causing the screen to blur. If they come into your path, destroy them with a ranged weapon and dodge the clouds of poison to avoid their nasty effects. Again, a side path leads to a switch that opens the gate, as well as a Skybridge that returns the Kid to the gate. However, another path that branches off of this one should be taken first, in order to pick up a Health Tonic and some extra Fragments.
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By the next gate is another side path that leads to a switch and a Skybridge, so repeat the now-familiar process to open it (the railroad tracks off to the other side of the path lead nowhere). Some Peckers that shoot needles at the Kid will swoop in to attack; defeat them by counter-blocking or by shooting back at them with a ranged weapon. After opening three more gates with switches, you will have to take a Skybridge to reach the next switch. Past this gate, break some logs to reach the next switch, which opens the final gate. Along this path is an important memento, the Hidebound Journal, and past this memento is the Singer. Speak to her to return to the Bastion, then speak to her about the mementos that you've collected thus far.