There are a few places I can think of. In Index, is a place called Reiter pit. It is off of Reiter RD. Most of the locals in Index, sultan area would know what and where the pit is. mkae sure you visit the "Index Wall". Ask any mudder what that is. Also, another is Walker Valley. I believe that is closed right now though. That is north east of Arlington. A map will have that listed on it.