Answer the following question given this paragraph: 

Tar sands are rocky materials mixed with very thick oil. The tar is too thick to pump, and so tar sands are strip-mined. Hot water and caustic soda are used to separate the oil from the rock. This creates a slurry, which is shaken. The oil floats to the top and is skimmed off.


Q: Tar sands are rocky materials mixed with what?


A:
very thick oil