Right after leaving the cave, you'll see the hanging crate with ammunition - of course, pick it up. The move down slope (between rocks).
You'll reach the wide area, where a few small dinosaurs will attack you. Shoot them down with sniper rifle.
Don't move farther, but kill one of the 'bats' hanging under the branch. His fall will attract some monster. Start killing them, but be ready for some enemies running in your direction.
Move down, where you'll again be attacked by small lizard. Try to get rid some of them with spear, to save some ammo.
You should hear Jimmy's calling you nearby.
Walk ahead, taking narrow passage between rocks (two beasts to defeat).
At it's exit you'll find yourself in front of the boy (he's standing on small island, on the other side of the river), who will be attacked by big number of dinosaurs. Pick up ammunition quickly and safe him. Some seconds after, enemies will try to kill Hayes again. Help him (only little), because the next wave will hit Jimmy.
After the fight, the boy will enter one of two life rafts and flow along with the current of the river.
You should move down between rocks, jump into water and move ahead. If you look round well, you'll notice running natives (hiding).
Leave water to reach the river's other side and move deep into the land.
Going through the narrow passage, the gate will close right behind you. The only way out is the gate on the glade's end. After a few seconds, it'll start opening 'itself' - it'll be an ambush. Natives send some dinosaurs against you. First, two waves of small reptiles (take advantage of spears and sniper rifle). The next, bigger monster will show up. Use your firearms to shoot him, when he's still far away.
Move ahead, where natives will attack you with burning spears. Pick up one.
Walk by the river and find Jimmy. He'll stop on the other side and wait, until you'll join him. Get there quickly.
To do that, head up and set the shrubbery on fire (but be aware of attacking natives - you can hide squatting behind small rock).
Run over the root (?) located above water and head to the raft waiting for you. Many burning spears will be 'chasing' you now and some lizards will show up from the meadow on the left, but don't attack them.