i drilled 4" into the base and 4" into the large red rock and inserted a 1/4" rod cut to length for added stability. then drilled through the flat one and into the red one above a couple inches and inserted rod. drilled into the top and through the first black grey rock. you need to be very careful when drilling as you will break out the back of the rock, so you go through half way and come back at it from the other side and hope that you meet in the middle. had perfect holes for these luckily. there is no glue used as the 1/4" drill bit is a nice tight fit for the rod. each hole in the rest of the rocks is 1 1/2" deep or so, wind will not move these around.
i just hold the rock down in a bucket of crushed stone and hope it does not move much while drilling with the other hand and hope the holes are true. if the hole is off a tad you can bend the rod by tapping it with a hammer to offset any small difference.
you cannot see any of the rod, using the right shaped rock to match the one below it insures that it is invisible.
