The Rock Bridge used to serve as a way to access the Swamp in a vehicle, but, the bridge no longer works, and this cannot be done. The bridge used to be colored light brown in earlier versions of the game, but now it has the same coloration and material as the surrounding Rocks and Cliffs.
The rock is about the size of one Snow Arch. Under the bridge is the Rock Bridge Cave, which leads to the widely-speculated Shrine of the Sight which recently has been solved using two numbered cubes.
The bridge no longer works after multiple attempts on different servers.
The Rock Bridge had one main purpose: to get vehicles to the Swamp. The player used to be able to first bring dynamite through the Volcano and continue forward, carefully off of the side of the volcano (the side that faces the Safari). Then, the player could place and detonate the dynamite at (most preferably) the middle of the rock. The rock bridge would then slide down over the Rock Bridge Valley, just over the Rock Bridge Cave. This no longer works as of July 8th, 2018 - detonating dynamite near the rock bridge will not cause it to move at all (but doing so will cause it to temporarily act like a very large Conveyor, pushing objects touching it in a single direction).
If the Rock Bridge had slid down over the Rock Bridge Valley recently, the player could then bring their vehicle down to the Volcano entrance, hook up their trailers, and continue to the swamp. The player would need to stick to the gravel road until it came to a stop. From there, the player would need to look to the right for a small ramp that leads down the cliff. Then, turning to the right at the bottom of it, the player would then come across the Swamp Cavern and, after passing through it, would then arrive at the Swamp.
After collecting desired wood and/or materials, the player would then drive out of the other end of the swamp and follow a smooth mountain valley that leads to the Safari. Alternately, they could use the bridge to go back the way they came. This bridge tended to glitch a lot, so, even before it stopped functioning entirely, it was not the recommended way of getting to the Swamp. It is to be noted that after several minutes, the bridge would crumble into little segments and despawn, needing to be blown up once more once it respawned in its starting location. Also, it was not recommended to put dynamite on the bridge after it had been blown away from its initial location --- the force of the explosion could fling the bridge off its resting spot.
On October 29, 2016, some people had figured out that by jumping or shift-locking at the bridge, it would eventually roll off. This revealed that the rock bridge was unanchored and the force of the dynamite, not a script detecting detonation of dynamite, pushed the bridge down.
As of September 16, 2017, the bridge no longer responds to dynamite detonation. It has been confirmed on multiple servers with multiple attempts.