In 1885, after rescuing Clara from falling into the ravine and saving her life, Doc and Marty realise that she was meant to have fallen into the ravine hence it being called 'Clayton Ravine' in the future. However, when they discover the Doc's tombstone in 1955 it is engraved "Erected in eternal memory by his beloved Clara". Since she only ended up not falling into the ravine and being alive AFTER Marty went back to 1885 wouldn't it be also true that the Doc wouldn't have known her before Marty went back hence her name would not have appeared on his tombstone?