One of Grindstone's foals, Birdstone, upset Smarty Jones to win the 2004 Belmont Stakes. Birdstone, in turn, sired Mine That Bird, a gelding that won the 2009 Kentucky Derby, and Summer Bird, a colt that won the 2009 Belmont Stakes. Empire Maker, which won the 2003 Belmont Stakes, sired Pioneerof the Nile, who placed behind his cousin Mine That Bird in the 2009 Kentucky Derby, and eventually sired American Pharoah, the first-ever winner of the Grand Slam of Thoroughbred racing in 2015. American Pharoah gave Unbridled the honour of four generations of Triple Crown race winners.