The World War I Somme American Cemetery and Memorial in France is sited on a gentle slope typical of the open, rolling Picardie countryside. (Photo by Warrick Page/ABMC) As the United States celebrates its 250th anniversary of independence, a group of Gold Star mothers have embarked on a trip to Europe to collect "Sacred Soil" from ground made hallowed by the courage and sacrifice of U.S. service members who fought and died in France and Belgium…