Gravel was a type of rock found on Earth.
According to the newly regenerated Romana II, concrete was created by adding gravel to cement. (TV: Destiny of the Daleks)
Reegan dumped the bodies of two of his thugs in a gravel pit. (TV: The Ambassadors of Death)
Behind the scenes
Many Doctor Who stories, such as The Tomb of the Cybermen, were filmed at gravel pits.