Nancy drew girl detective writ in stone [graphic novel]

It's double trouble for America's favorite girl detective when Owen Zucker, a sweet young boy Nancy has often babysat, is missing and a shore stone marker found on the coast of California, which may prove the Chinese discovered America in 1421, before Columbus, is stolen! Will Nancy, with the help of her best friends, Bess and George, be able to solve two baffling mysteries at the same time, while all of River Heights is watching?