I came across aerial imagery of a dirt strip in northern Idaho with a white "X" at one end of the runway, but no markings at the other. Does this mean the runway is only closed in one direction, or that it's fully closed and one of the markings is missing?
I think the X does mean that it is closed in that one direction. If you look at the 3D view you will see that there is a mountain off the approach end of the X runway making landing on that runway virtually impossible. If you were to depart on the other non-X runway you would have to turn relatively quickly to avoid hitting the mountain.
The X closes that approach end. The other end of the runway is considered usable. Also, it may be nonstandard, but I have seen X used with a new threshold for a temporary displaced threshold.