A ________ is a line of rocks or sand just above or just below the surface of the sea, often dangerous to ships.
reef
A ________ is a hill of sand near a beach or in a desert.
dune
A ________ is a large valley with very steep sides and usually a river flowing along the bottom.
canyon
A ________ is water, especially from a river or stream, dropping from a higher to a lower point, sometimes from a great height.
waterfall
A ________ is a group or line of mountains with a particular name.
mountain range
A ________ is a mountain with a large, circular hole at the top through which lava gases, steam, and dust are or have been forced out.
volcano
A ________ is a large mass of ice that moves slowly.
glacier
