North Fork Cave would have been formed as part of an ancient underground water system. When the canyon eroded out it exposed a portion of this system. It really does not have anything very interesting to see inside, it is simply a pocket about 150′ long and about 40 ft. tall at the entrance.
Some of the treasure stories in North Fork refer to the natural caves as a point of reference.  If you can find this one then you may be on your way to solving the other mysteries the canyon holds.