I am building a garden gate and frame out of PT lumber. The gate
frame is two 4x4s that hold a 4x4 cross member as so:

| |
| |
| |_____ground level_______
|----|
| |

How can preserve the underground members so that they will last
longer? I will be putting a layer of gravel and some cement. Is there
a product such as cresote or tar that I can apply before I install the
frame?