Bottlebrushes are formed when stalactites become immersed in a cave pool for a long period of time. For this to occur, a change in waterflow into the area beneath the stalactite had to occur to produce a pool. This often accompanied changes in climate which brought more rain, such as during the Pleistocene. Rather than continuing to grow longer, the stalactite will become coated with pool spar if the pool is supersaturated with calcite. The left photo shows a classic example of a bottlebrush from a California show cave, Black Chasm Cavern. The righthand is from Lechuguilla Cave, New Mexico, and also shows a coating formed in a subaqueous environment, but in this instance mammilaries rather than spar crystals have formed the coating. The two images on the bottom show former pool basins where both stalactites and stalagmites have become coated with spar deposits, from Nevada and New Mexico.
Other names sometimes used for these types of formations are war clubs, pendulites, or lion's tails. In fact the area where the photo on the bottom right was taken is called the War Club Room.