Rock Creek is in Georgetown California. I went there for the first time last weekend and it was some of the toughest terain I have ever riden. What they called a blue trail was more like a black diamond. Lots of single track and goat trails awsome stuff. The park is beautiful with 45 miles of trails and some water crossings. I would say it is the best place I have had a chance to ride in NorCal and it is only one hour from my house. Camping is allowed with know fees and red sticker bikes are allowed all year. The terain is rocky in some areas run 14psi at minimum and you will not go higher than third gear so you might want to change to a higher than stock gear ratio. Carry a camel back and tool kit I needed both and GPS would be great but the local ranger station can sell you a map for $3.25. The rangers are very nice but once you are on the trail you are on your own so go prepaired.
California ATV & Dirtbike Riding Places (Georgetown)
510-333-4312 (Ranger Station)