Red Car Canyon: Mystery Walnut, Perhaps Not

Photo: Diane Edwardson, June 12, 2012.  For the past 2 years, we've tried to identify this tree.  It's not Southern California Black Walnut; this tree has much larger leaves and fruit.  (Click on photos to enlarge.) 
The squirrels always go nuts for this fruit.  It's 3 times the size of the native Southern California Black Walnut.
A reader from Northern California suggested it might be California