The Petrified Forest is in the second set of tracks in this DLC, Sixty Six Pros, and you'll get the squirrel for this track while going for this achievement.
For this one, reverse from the start to enter the underground caves. You need to perform a series of drops down towards the bottom of the cave, correcting your balance so you land safely. All the drops are the same distance, apart from the final one.
So, from the checkpoint in the cave, reverse and drop down. From here, go forward and drop down. Then reverse and drop down. Go forward and drop down. Go forward again and drop down. Then reverse and drop down. Then finally, go forward for a bigger drop to the bottom of the cave, where you'll find the squirrel. Simply ride forward from there out of the cave and the achievement should then pop.