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Stalactites stick "tight" to the ceiling....
Stalagmites "grow" up from the ground...
Features such as stalactites and stalagmites are commonly called cave formations. The general term for any mineral deposit or formation found in caves is speleothem, derived from the Greek words "spelaion" (cave) and "thema" (deposit). Calcite is dissolved from the limestone above the cave by slightly acidic water as it percolates down through the soil. When the water reaches the cave, it redeposits the calcite, and forms a stalactite or other speleothem.
Speleothems form at varying rates as calcite crystals build up, one upon the other. Although it takes an average of 120 years for a cubic inch to form, several factors can determine the rate of growth. The shape of speleothems is determined by how the acidic water enters the cave (by dripping, seeping or splashing) and how the water stands or flows after entering the cave.
Some common speleothems include:
Aragonite Needles: a cave mineral, which often forms needle-like crystals.
Cave Coral: also known as "popcorn"; small knobby clusters formed by seeping water.
Column: when a stalactite and a stalagmite grow together from the floor to the ceiling.
Drapery: a speleothem formed when water deposits calcite in thin sheets that hang in delicate folds.
Flowstone: resembling frozen waterfalls, form when water flows down walls, over floors and older formations, building up sheets of calcite like icing on a cake.
Helictites: small twisted structures projecting from ceilings, walls and the floor of caves that seem to defy the laws of gravity. Formed by seeping water, they project at all angles.
Rimstone Dams: steplike terraces along streams and on cave floors that enclose pools of water.
Shields: flat semi-circular calcite sheets formed by water seeping from a thin crack. Stalactites and draperies commonly decorate the edges of these rare speleothems.
Soda Straws: thin hollow tubes that grow from the ceiling of caves as water runs down inside them and deposits rings of calcite at their tips.
Stalactite: an icicle like deposit that grows down from the cave ceiling. Stalactites form as calcite is deposited around the outside of soda straws after the centers of soda straws become plugged.
Stalagmite: speleothems that grow up from the floor; usually formed by dripping water from stalactites above. They are more rounded on top than stalactites.
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