Learn who’s buried beneath the gravestones located near the Elmwood Avenue entrance to Roger Williams Park. Saturday, the Museum of Natural History and Cormack Planetarium will offer a cemetery walk of the Williams’ Family Cemetery. Museum docent Richard Siembab, a longtime member of the Association of Gravestone Studies, will lead the tour and explain its history, including the meanings of the symbols on gravestones and the relation of those buried to Roger Williams, the founder of Rhode Island.