Thanks sergebmw3!
@Smann - I'll put up some closer pics but there are acrylic rods going up through the 2 columns, they are glued to a flat acrylic base that is sitting on the glass bottom- this also made me feel better in case something fell or if a rock were to drop onto the bottom. So I just drilled through the rocks to fit the rods, there are also a couple zip ties helping with the rocks making the horizontal bridge.