There must be some decaying trees and brush at several points in my
yard. This has resulted in gullies that feel terribly unsafe on my
tractor/mower. So, I want to fill them in to level the lawn that will
be planted over the fill.

Should I use sand or 1" gravel to fill the low spots? How much top
soil should I put on top of that to grow the grass in? Will the new
top soil wash into the fill requiring more soil? If I figure out how
much fill is needed in cubic feet, how does that relate to tons of
stone/sand? Some one told me fill is sold by the ton rather than by
the yard.