Hm, those cliffs don't seem very efficient for an environment. Rather than having a couple large low res chunks, it's better to have lots of smaller hires chunks (depending on the situation)
For Uncharted, they made a variety of rocks and then stacked them in the scene. You could probably follow the same approach
The thread: http://www.polycount.com/forum/showthread.php?t=98332
Otherwise, looks like you're off to a good start!
At the block in stage you're at now, try matching the camera angle of the concept and see if yours is to the right scale and stuff