Head away from the stele and around the sphinx's right paw to the west, where there's another deep pit.
Take a running jump over the steep sloping edge on the left (south) side of the pit to land on the flat ledge near the square, stone columns. Turn right (north) and move toward the edge of the pit.
On the far side of the pit, to the north, 2 more assassins lie in wait.
You can take them out from here using the crossbow with laser sight and an explosive arrow or two. (Or, to conserve ammo, use regular/poison arrows or pistol ammo with the laser sight bug.)
Now you can take your time crossing the pit: Start by sliding down onto the triangular ledge in the southwest corner of the pit (just below and to the left of where Lara was standing in the last two screenshots).
Move to the far point of the triangle and take a standing jump to the next ledge, which juts out from the left side of the pit.
From there, take a carefully angled running jump to the rectangular ledge in the northeast corner of the pit.
Climb out of the pit here, near the big doors the assassins were guarding.
To the left of the doors you'll find either a breakable crate containing a WOODEN HANDLE or the handle itself (if you blew up the crate when you killed the assassins). Pick it up.
Turn around and climb onto the square block near the sphinx's paw. Take a standing jump up onto the paw. Turn right and run to the end...
...to get some wideshot shotgun ammo. Then slide down to the ground near the white stele.
Combine the METAL BLADE and WOODEN HANDLE to make (ta-da!) a SHOVEL. Use the shovel in the cracked earth in front of the stele to start digging.
After the cutscene, drop into the hole and follow the tunnel to end the level.