This cache can help with several challenges. Rhyme Time Challenge, California Multi Cache Challenge, and maybe more.
This is a simple two stage multi. GZ should take you to a piece of vertical truss from the old Carquinez Straits bridge. On the south end of the truss is a small plaque. Find the year of the old bridge. Add the 4 digits of the year together to get (X). On the same face (side) of the truss as the plaque there are 4 rows of rivets, count the number of rivets in the 4 rows (Y).
N = -(X+Y)*10+798 and W = (-X+Y)*10+340
Final is at N38 03.N and W122 14.W.
Check sum for all digits = 44