The trails northeast of Roslyn have an abundance of amazing geological features. This is a short hike from town which will take you past many of my favorites. Heck the trail itself is solid rock for some sections. The cache is nestled inside a particularly large specimen. I could only get a marginally accurate location reading, but geosense should lead you to the prize.
Congratulations to NetFire, DrSalami and SecretVista for the FTF!