For this you must knock over all 16 rock cairns (rock piles) with your car in chapter 10. Simply drive over them and they will fall apart.
All of them are in chapter 10. At certain points in the mission you cannot go back. This means you must get them in the correct order. You can't drive back from the last cairn to the first, you would have to replay the chapter for that. The cairn locations are as follows:
#1 - In front of where you start.
#2 - In the bigger area shortly after the start (area with waterfall on the left).
#3 - Same area as previous cairn, but at the end of the area (where you drive up the mud).
#4 - After driving up the mud, before the wooden bridge on the left.
#5 - After crossing the wooden bridge on the left.
#6 - Shortly after passing the bridge you drive up a muddy hill. After that you come to a new area with a small river and you must drive up a large rock. On the rock is a cairn.
#7 - After driving up the second rock on the right (same area as previous cairn).
#8 - You'll drive down a muddy hill and see some ruins in front of you. Before the ruins on the left is a cairn.
#9 - After the last cairn you jump down a little rock and come to a relatively small circular area. At the end of this area is a cairn.
#10 - In the area after destroying a wooden bridge with the winch. When Sully talks about Nadine.
#11 - In the very large area on your way to the beacon. It's at the start of that area, after you see some trucks driving away in the distance.
#12 - Same area as before, further up the hill
#13 - Same area as before, on the hill on the left side with the manmade sculptures.
#14 - Same area as before, near the top of the hill (but before the beacon). This is the best hidden one, it's standing in some tall grass. Hard to see. It's just before making a right turn to the hilltop with the beacon. To the left of it is a path that leads to a village with enemies.
#15 - On the hilltop where the beacon is located. In front of the beacon.
#16 - After the second beacon / tower, on the right side of a cliff. You'll drive past it, hard to miss this one.