A volcanic mesa directly above the town of Golden was formed by an ancient lava plug that remained after softer surrounding material eroded away. The standard mesa loop (Rim Rock Trail, Fossil Trail, Old Quarry Trail, Basalt Cap Loop) is approximately 4.6 to 5.9 miles. The full park trail network spans 16.1 miles. New T. rex Tooth Trail creates a new 3-mile loop when combined with Fossil, Old Quarry, and Basalt Cap. Tip: Castle Rock, the prominent Golden rock outcrop, is on private property; do not leave the trail to access it. Second: Rattlesnakes are frequently encountered. Stay on the trail and watch where you step.