One of the best beaches we have found in our extensive sand castleing research has been just south of Corpus Christi, TX.  Over the years, the Mississippi river has washed tons and tons of fine grained silt from the heartland of North America into the Gulf of Mexico.  Ocean currents push it this way and that before some settles on the beaches of Corpus Christi.  While the rich dark gray color adds to the imposing nature of the Gothic-Gaudi-esque structures, it is really the delicate arches, spires, and walls allowed by the minute sand grains that wows the passers-by.