Rock Mary in Caddo County, Oklahoma, became a noted landmark on the California Road, the southern route to California, because it stood out across the open prairie and helped westbound emigrants stay on course. On May 23, 1849, Lieutenants J. E. Simpson and M. P. Harrison visited the site, raised the United States flag on its crest, and named it for Mary Conway, the 17-year-old daughter of Dr. John R. Conway, who was traveling to California with his family. Captain Marcy also valued the spot for its nearby spring, campground, and wood, which added to its usefulness for travelers.