Gravel

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.