stalactite v. stalagmite

Photo by Richard Cindric Stalactites and stalagmites are types of speleothems (cave formations) created by the accumulated precipitation of minerals (usually limestone) dissolved in water. Stalactites are pointed structures hanging from cave ceilings. Stalagmites are rounder mounds that grow up from cave floors. When a stalactite and and its corresponding stalagmite grow long enough to join one another, the resulting speleothem is a column.

Wikipedia on speleothems

StateMaster.com on stalactite/stalagmite mnemonics