One is to manually focus, but being that the light level is low and the detail is minimal, that will also be difficult. Another method is to just estimate the distance and set the focus on the lens using the distance scale and selecting an aperture that will allow for a margin of error with the DOF. Unfortunately Nikon no longer includes a distance scale with newer designed consumer lenses (what were they thinking!!!).
Another suggestion I read is to carry around a flashlight (I believe you Brits call it a torch). Just shine the flashlight on the rock to attain and lock focus, then recompose and shoot.
Also the focus assist light on the camera has a limited range, using a hot shoe mounted flash such as the SB700 would allow a greater distance with its IR based focus assist lamp.