You need to find 3 wheel cogs to fix a machine which will open the way to the next level, as well as the shotgun.
Walk until you come to an open area with a stream dead ahead. Walk to the right and you’ll reach a series of rocks that lead down to a body of water, around which two wolves are prowling. It’s easy to pick them off from a safe distance. Climb down to the bottom and find a cave to the right of the stone steps (screenshot), opposite the waterfall. Here you’ll find another wolf; kill it and continue on until you come to a wall with an opening above it. To the right is another passage containing three wolves, but unless you’re trying for all the kills, there’s no need to go down there.
Climb up to the opening and keep going until you come to a skeleton and a large medipack. Retrieve it and continue on until you reach the entrance to the valley. This is where it gets dangerous! Drop down and listen carefully and you’ll hear the noise of an animal getting nearer! Then you’ll see a velociraptor charging towards you! Run back to the rocks and climb up so you can shoot it in safety, or just jump around as you shoot...they are not too difficult to kill! As you proceed into the valley, you’ll encounter yet another raptor.
Kill it then start walking further into the valley. You’ll hear some very loud footsteps ahead of you! A huge T-Rex appears, and to shoot him in safety, you can run into a cave set in the right wall near the broken bridge (screenshot) and shoot the T-Rex from there as he paces back and forth! You can also take your chances out in the open, taking care not to get cornered by it or letting it grab Lara in it's mouth! Afterwards, if you want all of the kills, run though the aforementioned cave to kill another raptor.
When that’s done, and you’ve calmed down a little, continue forward and drop down from the opening and keep walking across the valley. On your right, you’ll pass a waterfall and stone archway. Go under it and pick off two more raptors. Walk towards the building, but before you get there, look to your right and you’ll see a climbable rock (screenshot). Pull up onto it and turn left. Walk to the front and slightly to the edge of the ledge you are on, jump over in a diagonal direction to jump over to the ledge beyond the angled block and then pull yourself up. (This is a tricky section, so it may take more than one attempt to get it right.) Look slightly to the left and see an angled rock with a greenish substance on it. Walk to the left of the ledge you are on and jump to grab the edge of the green rock. Pull up and walk to the left-hand edge again. Take another standing jump to grab the high rock ledge. From there, take a running jump across to the roof of the building. This is the first secret, and it’s a good one, because if you look around you’ll find a large medipack, Magnum clips, Uzi clips and some shotgun shells. When all of these have been collected, you can either go back the way you came or go to the far end of the roof and jump over to the back wall of the valley where you will slide down to the ground.
Go around to the front of the building and enter, where you’ll find a save crystal. You can wait until you’ve found the first cog in the water before you save your game if you like, but there’s another save crystal not far ahead. Swim down and to the right to find it down an underwater passage where you can pick up the cog. Come back, climb out and save your game now, if you didn’t before.
Exit the building and look forward and to the right. You’ll see a lit cave in the wall. Climb up to it and follow the passage to the broken bridge. Here you’ll find another save crystal, and you can either save it here or wait until you have the second cog which is across the bridge. To get across the broken part, do a run, jump and grab. Collect the cog and then save your game if you haven’t already done so, and safety drop from the bridge to the ground.
As you are facing the broken bridge, with the waterfall to your right, look to your left and you’ll see an opening almost hidden by vegetation (screenshot). Climb through and follow the passage. You’ll end up on a rock above another raptor which you can kill from where you are. Drop down, follow the passage and climb up the rocks to the right of the pool of water. At the top is the third cog. Collect it and then climb back down, jump into the pool and swim down the passage until you find yourself back in the valley.
Climb out, turn around and you will see a waterfall above the pool out of which you climbed. To the right of this is an opening (screenshot). Climb up to it to find the second secret, some shotgun shells. Go back out and look somewhere between the two waterfalls. There are some climbable rocks (screenshot )…climb up and jump forward and to the right to a ledge*. Walk forward and then jump up to grab the crevice in the wall and shimmy to the right by keeping hold of 'action' and pressing the right direction button. Pull up into the small cave above the waterfall. The cave may appear empty, but head to the back and to the right to find the third secret, Magnum clips and shotgun shells. Climb back down to the ground.
For the next part of the level, you need to make your way back to the beginning, where you first entered the valley, past the skeleton. Climb all the way back up the rocks from where you shot the first two wolves and continue along the stream past the opening at the start of the level. To head upstream, you’re going to have to jump from bank to bank, and if you miss, you’ll find yourself being swept over the waterfall into the body of water where you shot the wolves, and you’ll have to start all over again! All of the jumps are run and jumps, and you need to make sure you angle and line up carefully for each one!
Follow the passageway to the bridge and use the save crystal to save your game. Across the bridge is some machinery with 3 missing cogs. Place your cogs in the spaces by standing in front of them and pressing 'action'. Then pull the switch nearby to start them turning. This machinery moves a gate to shut off the water to the waterfall. Walk to the right edge near the bridge, turn around and drop and grab the edge (screenshot). Shimmy left and drop onto a flat rock below. Enter the cave to find the fourth secret, a large medipack.
Jump into the water and swim down to the golden gate. Turn left and climb out onto the right bank, relieving the skeleton of the shotgun before jumping back into the water and swimming along the narrow passage. Come up for full air before diving back under and swimming through a long, narrow passage to a small room through a hole in the ceiling where you’ll find the fifth secret, a small medipack and two lots of shotgun shells. Go through the doorway, safety-drop down, turn right, and jump over the slope into the water far below! (Hint: to dive instead of jump, press 'jump', 'forward' and 'walk' one after the other in fast succession!)
Find a ledge behind the now dry waterfall and climb out to follow the passageway to end the level.
*One time when I jumped to this ledge, I got stuck inside the rock and couldn't get out! I could turn around and shoot my weapon, but I couldn't walk, jump or roll so had to reload my game!
If you missed anything while playing the PC version, here is the savegame file with all kills, pickups and secrets.