Stratum
From Tardis Wiki, the free Doctor Who reference
A stratum was a layer of rock. Blasting work carried out in an English quarry brought a one hundred and fifty million year old stratum to the surface. Within it was the hand of Eldrad, which Sarah Jane Smith discovered while trapped beneath the rubble of the explosion. Ammonite shells were often found in stratums of that age. (TV: The Hand of Fear)
