You are a US citizen and: 1  You are a first time applicant age 18 or older. 2  OR your most recent U.S. Passport was issued when you were under age 16. 3  OR your previous U.S. passport was issued more than 15 years ago.
Cave formations are called speleothem, from the Greek word spelaion, cave and thema meaning deposit. Almost all of them are made of calcite, the crystal of calcium carbonate. Different minerals and different movements in the water account for an incredible variety of cave formations.
Either way, because the soluble limestone is partially dissolved and carried away, these are a type of solutional cave. Less common solutional caves are formed in other types of rock like chalk, marble or gypsum. Often limestone caves are named caverns.hey're in many areas of the world, so you have a lot to choose from when planning your next adventure. The USA has many limestone caves and caverns. Below you will find a state by state listing of the most popular. Click on the ones that are links for more information.