File:Coralloids (knobstone) (Endless Caverns, Virginia, USA) (31263442075).jpg

Travertine coralloids in a Virginia cave, USA.

"Cave formations" in caves are technically called speleothem. Most speleothem is composed of travertine, a crystalline-textured chemical sedimentary rock composed of calcite (CaCO3). Travertine forms in most caves and at some springs by precipitation of crystals from water. Pure travertine is white. The colors seen in the travertine shown above are from iron oxides. Travertine speleothem occurs in a wide variety of forms. The most common variety is dripstone, which forms by the action of dripping water - examples include stalactites, stalagmites, columns, and draperies. The second most common variety is flowstone, which forms from flowing films of water. Flowstone has the appearance of a frozen waterfalls.

The travertine speleothem shown above is in Endless Caverns in Virginia. The pustulose to nodulose structures covering the stalagmite are knobstone (a.k.a. coralloids; a.k.a. "cave popcorn"). There are multiple proposed origins of knobstone, including: 1) water seeping through cave bedrock and through cave popcorn itself 2) thin films of water flowing over wall irregularities 3) splash from dripping water 4) water moving by capillary action from pools into walls 5) condensation water 6) aerosols

The publicly accessible portions of Endless Caverns are developed in structurally tilted carbonates of the New Market Limestone (Middle Ordovician). Other portions of the cave are in overlying units: the Lincolnshire Limestone and the Edinburgh Formation.