It is located in front of a large 'peninsula' of rocks that jut out, making a bend in the creek. The initial rock is about three players high and five players long. The rock is brown, with a slate material, like any other rock on the Mountainside and Swamp.
The rock gives off an awkwardly increased amount of steam than the rest of the Swamp. The Swamp Rock is likely a plug for the hole into the river tunnel, which may serve as a secondary way of accessing the tunnel if it is finished.
Prior to an update somewhere in November 2015, the player could have used Dynamite to blow up the Brown Rock off of the Pink block in order to access this. This was tested and proven to be fixed as of January 2, 2016.
However, during the time the player could access this block, it had some unique attributes. While the block is not affected by Dynamite directly, any loose item that came in contact with this block could not be interacted with anymore.
Interestingly, according to a leaked save of the game, the Swamp Rock is named as "BoulderRegen", and even has a script that regens the rock after a certain time it has been exploded. But as said above, this cannot be blown off anymore.
Even more, the pink block is called "Blocker" meaning that this was used to block players from going down the hole to the river tunnel. However, this is not needed because the boulder is named 'NOTA_Boulder" while the script only looks for a part named "Boulder" meaning that the Swamp Rock cannot be blown up.