Marty McFly searches for his father's tombstone in Oak Park Cemetery in 1985A.
Marty stumbles across Doc's tombstone in the Boot Hill Cemetery in 1955.
Tombstones are markers for a particular grave to indicate who is buried beneath, and often their dates of birth and death, family relationships and notoriety.
History
In 1985A, Marty McFly found out that his father was killed and buried at Oak Park Cemetery. Upon going there, he discovered the tombstone, which was smaller than most of those surrounding it, confirming that George had indeed died in 1973.
Later on November 14 in 1955, Marty and the residental Emmett Brown prepared to leave the Delgado Mine only for Copernicus to be troubled by a grave at the nearby Boot Hill Cemetery. Both Marty and Doc looked at the tombstone to discover that it was the grave of the 1985 Emmett Brown, and took a photograph using Doc's camera.
After having arrived in 1885, Marty looked at the picture of the tombstone to keep track of his progress. Upon looking at the photograph after preventing Buford Tannen's shooting of Doc, it was revealed the name and date of death had been erased from the tombstone, but the marker's presence suggested that someone would be buried there — either EMMETT BROWN or CLINT EASTWOOD.
During his fight with Buford, Marty attacked him with the yet to be used tombstone, shattering it in the process. After winning, Marty took another glance at the photograph, just in time to see the image of the tombstone vanish before his eyes.