Well that's part of Chapter 3, now it's not easy for me to tell you what to do since you can do chapter 2 and 3 in whatever order. So It kind of depends on whether or not you've done chapter 2.
If not then you can access chapter 2 through the waterfall area.
If you've done both and need chapter 4, it's in the area near the merchant house where you can now clear the rubbles.