In 1973 and 1975, doughnut-shaped stones which resembled stone anchors which were used by Chinese fishermen were discovered off the coast of California. These stones (sometimes called the "Palos Verdes stones") were initially thought to be up to 1,500 years old and therefore proof of pre-Columbian contact by Chinese sailors. Later geological investigations showed them to be made of a local rock which is known as Monterey shale, and they are thought to have been used by Chinese settlers who fished off the coast during the 19th century.