The first thing you have to do in order to continue, is to destroy the wooden construction blocking the way.
Then, go through the split in the rock. You'll reach a broad 'meadow' in which a lot of creatures live. Stand on the elevation on the right and kill the creatures by throwing spears at them (when you're out of spears, use the bones that Ann brings you). You can kill them faster by knocking down the containers full of fire onto the bushes (with the sniper rifle - like in the picture - or with spears).
Remember that lighting the bushes on fire won't eliminate everything so once you're down there, you'll have to fight.
Take advantage of the moment, in which the creatures will eat each other - that's the key to an easy victory (on the picture, a flying creature eats a 'beetle' - don't only use the sniper rifle, but also spear).
Once you're on the bottom, try using your primitive weapons, since there is a lot of them (you can use one spear multiple times by removing it from an already killed creature). Don't forget to pick up the ammo from the crate.
Once all dangers have been eliminated, move on the only available road. You'll walk through a narrow, twisty tunnel until you get to a spot, at which you'll have to jump off the elevation. That is where you'll see for the first time (look at the wooden footbridge above your head) a native.
No one will attack you there, so follow Ann without any worries. At first, through the first bridge...
...and then next, until you'll reach a 'round' passage in the rock.
And again, you'll reach a large area, in which you'll fight with a large number of monsters. Use bones to exterminate them, since someone's remains will lie there.
Once you think you've killed enough monsters (from the distance - I advise killing most of them), make a passage by burning the bushes with fire from the wall on the right.
Lead the next fights from close by (just watch out for attacks from behind) and remember that you can use one spear multiple times by removing it from the bodies. Also, don't forget that you can light up the remaining bushes.
Further on in the meadow, there will be a flying opponent. Kill him when he'll be eating another creature.
When the area is cleared of all unfriendly life forms, you can continue the adventure by following Ann onto the elevation.
You'll reach a spot where Ann will climb a wall onto a rock shelf, and you jump into the water. She will open the gate for you.
Walk a short distance on land and jump into the water again. This time opening the gate will create a close meeting with unfriendly bushmen. You'll be attacked and you'll lose consciousness.