A six-foot sketch by the English Romantic painter John Constable will lead Christie's 250th Anniversary sale on June 30 in London. The study for a painting in Constable’s Stour Valley series (the finished work hangs in the Huntington Library, Art Collection and Botanical Gardens in California) is expected to fetch around $18 million-$23 million. The oil on canvas has been in a private collection for the past twenty years.