As someone pointed out, 18" deep for the 4x4 posts might not be enough to handle the high winds of Colorado. So I am now at least 6" under grade for my lower "deck" supports, and I increased the depth of the post holes to 24" (plus the 6" below grade). The most annoying aspect of this decision was dealing with the immense number of roots and the sticky clay that I had to manually scrape off my post hole digger blades each time I brought them up. At least my shoulders are stronger now!