Among the mysteries of how ancient people created structures to mark the solstice and equinox with astonishing accuracy, this one is central: How did they determine the dates of those astronomical events? Houstonian Gordon Houston, who is pursuing a doctorate on this riddle, says he believes he has figured out the answer for centuries-old glyphs known as the Pictographs of Paint Rock, Texas. Found on a private ranch near San Angelo in northern Concho County, these approximately 1,500 pictographs on a limestone cliff include about a dozen that "had a solar interaction," Houston said. "Some were incredible." Houston will talk...