Easy
When trails are dry, 2-wheeled-drive vehicles OK but high-clearance is needed. When wet, dedicated 4-wheeled-drive system is required.
Gravel, dirt, clay, sand, or mildly rocky road. Gentle grades. Water levels low except during periods of heavy runoff. Full-width single lane or wider with adequate room to pass most of the time. Where shelf conditions exist, road is wide with minor sideways tilt. Clay roads, when wet, can significantly increase difficulty. Some steep climbs require 4-low gear in the transfer case.
Moderate
A dedicated 4-wheeled-drive system with a 4-low transfer case is required. Rutted dirt or rocky road. Higher ground clearance required in some cases. Absolutely NO 2-wheel-drive vehicles.
Careful tire placement may be necessary. Some grades fairly steep but manageable if dry. Soft sand possible. Sideways tilt will require caution. Narrow shelf roads possible. Backing may be necessary to pass. Water depths passable for stock high-clearance vehicles except during periods of heavy runoff. Mud holes may be present especially in the spring. Undercarriage may scrape occasionally depending on ground clearance. Rock-stacking may be necessary in some cases. Brush may touch vehicle. Standard factory skid plates and tow hooks recommended on many trails.
Difficult
Modified offroad vehicles only. Lifts, differential lockers, aggressive articulation, and/or winches recommended in many cases. Skid plates and tow hooks required.
Grades can be very steep with severe ground undulation and large boulders. Sideways tilt can be extreme. Sand hills very steep with soft downslopes. Deep water crossings possible. Shelf roads extremely narrow; use caution in full-size vehicle. Passing may be difficult with backing required for long distances. Brush may scratch sides of vehicle. Body damage possible.