The kind of logging roads that I know up in North Western Oregon, Cannon Beach, will have crushed packed rocks of various sizes. Emphasis on sizes and that is why a full suspension bike is the best to have as much traction as possible--without back suspension the wheel will have a tendency to hop and therefore loose traction particularly on steep grades. Actually one of the road I like going up is called "Rock Crusher" and it is steep (18/20 degrees) with relatively small rocks.
Trees take about 30 years before they are logged/harvested and therefore the logging roads may not be in their best shape in areas where the trees are growing. They quarry rocks and lay and packed them on the roads, for obvious reason mud is not good for logging trucks.
It is great fun and you are away from cars, actually you may get far away from civilization, phone coverage and therefore help--so be well prepared.
The very few truckers that I have encountered have always been very nice and somewhat amazed a guy of my age being so far up, whenever I ear one I always stop and let him go because they are working. But they are rare and you learn quickly which area they are working. I don't use that bike any more (Klein) because I have a full suspended one (BMC)
