Sure. I don't have Photoshop available where I am writing this, so the following is from memory.
1) Add new "Levels" adjustment layer
2) Alter the histogram using the 3 arrow heads beneath it, until you think the rocks are light enough. I normally drag the left arrow to start of the histogram "hill" and the right arrow to the end of the hill. By moving the middle one, you should see the whole picture get lighter or darker. It is very much a case of just fiddling to see what you prefer here
3) With the Levels layer selected, goto the "Layers" menu and pick "Add Layer Mask", then "Hide All"
4) You should notice a white square has appeared next to the levels layer - this is the layer mask. Click on it to select it
5) Select the Brush tool, and choose black as your foreground colour. Now, wherever you paint on the layer mask, the new brightness from the Levels layer will show through - ie., paint over all the rocks with black.
This should show you the basics. The beauty of layer masks is that you can go back and tweak what is shown and revealed, simply by repainting on the mask. You can also double click on the Levels layer to adjust the brightness of the rocks when you have done the masking.
Hope that helps, and apologies if there any obvious mistakes
Matt