Smarter in sixty minutes.Get smarter in just 60 minutes with in60Learning. Concise and elegantly written non-fiction books and audiobooks help you learn the core subject matter in 20% of the time that it takes to read a typical book. Life is short, so explore a multitude of fascinating historical, biographical, scientific, political, and financial topics in only an hour each.Towering above Black Hills National Forest in South Dakota are four gigantic sculptures carved into the south-eastern granite face of Mount Rushmore. They depict the faces of U.S. Presidents George Washington, Thomas Jefferson, Abraham Lincoln and Theodore Roosevelt, who were chosen to represent the nation's birth, growth, development and preservation. Work on the project began in 1927 and was completed in 1941 under the guidance of the sculptor Gutzon Borglum. Some 450,000 tons of rock was removed to carve the enormous heads, each of which measures 60 feet in height. A small repository box is placed in a tunnel of the incomplete Hall of Records behind the sculptures; it contains 16 porcelain enamel panels telling the background and history of the monument. Mount Rushmore National Memorial, known as the "Shrine of Democracy," has become an iconic image of the US. As an international tourist attraction, it receives upwards of two million visitors every year.