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Dalmatas Cave
Creative Commons AttributionDalmatas Cave - Credit: BillBl, Flickr
cave or cavern is "a natural underground void large enough for a human to enter" although some claim that the term should be restricted to cavities of which some part does not receive daylight. In popular usage the words can apply to  sea caves, rock shelters, or grottos. There are various types of cave, formed in different ways:

The most common type of cave is the solutional cave, formed by the solution of rock in natural acid found in groundwater . Solutional caves are most common in limestone, but are also found in chalk, dolomite, marble, salt, and gypsum. Those in limestone often contain calcium carbonate formations; the products of slow precipitation, such as  flowstones, stalactites, stalagmites, helictites, soda straws and columns. Such formations are collectively known as  speleothems. Another type of solutional cave is formed when limestone is dissolved in sulphuric acid formed from hydrogen sulphide gas rising from below and mixiing with groundwater. The most spectacular example of this subtype of Solutional cave is said to be Lechuguilla Cave in New Mexico.

  Another type of cave is the  primary cave, formed at the same time as the surrounding rock. The most common primary caves are lava caves. Some of these are lava tubes,  formed when the surface of the lava has solidified and the hotter lava continues to flow under the crust,  afterwards flowing out and leaving a hollow tube. Lava caves also include rift caves, lava mold caves, open vertical volcanic conduits, and inflationary caves.

 The  Sea cave is yet another type, formed by the erosion of the sea on rocks. Some sea caves are littoral caves,  which result from zones of weakness in cliffs and from wave action. Others are solutional caves which have been flooded by the sea.

Other types are Corrosional or erosional caves, formed by erosion from flowing streams carrying rocks and other sediments. Such caves can exist in all types of rock, even granite and other hard rocks.

 There are also Glacier caves formed by melting, fracture caves formed by the solution of soluble materials such gypsum, talus caves (openings between rocks fallen into piles) and  Anchihaline caves, containing mixtures of fresh and saline water.

 Caves are also found under various different patterns. The different types of cave distinguished by pattern  include branchwork caves  formed by streams that meet as tributaries; angular caves made by fissures and fractures in carbonate rock widened by chemical erosion; anastomotic caves ressembling braided streams whose passages separate and then meet downstream; spongework caves- solutional caves joined by meeting of chemically diverse water; and ramiform caves formed as irregular large rooms, galleries and passages.

Public Domain"Arch-Druid in his full Judicial Costume" etching from Charles Knight, Old England: A Pictorial Museum (1845)

Druids were the priests, political advisors, teachers, healers and arbitrators of Celtic Britain. They were trained in "universities" and their knowledge was learned by rote. This knowledge was said to have been secretly taught in caves and forests. To become a druid was alleged to  take up to twenty years of study. Druids were entitled to speak before the king in council, acted as ambassadors and judges, and composed verse. They practiced animal and human sacrifice, and worshipped in oak groves and near water. The Isle of Anglesey was a centre for them.

Little is known for certain about the druids. Most of our evidence comes from hostile Roman sources. They were credited with magical powers and sometimes referred to as magi. The Greek scholar Alexander Cornelius Polyhistor called them "philosophers" and said that they believed in reincarnation. The "wicker man," in which men were said to have been sacrificially burned alive, is nemtioned in Caesar's Gallic War. It has been said that the druids could never have been associated with such stone circles as Stonehenge; because the circles were built in the Neolithic and Bronze Ages, long before their time: but this is equivalent to saying that the archbishop Rowan Williams could never have been in Canterbury Cathedral because it was built in the Middle Ages, long before his time!

The druidic practice of human sacrifice horrified the Romans. The druids of Anglesey were exterminated in 61 A.D., three years before the great Fire of Rome. Druidism then ceased to be a religious force in Britain, though it may have survived in Ireland as late as the 7th century. An attempt to revive it was made in the 18th century in the form of the Ancient Order of Druids, which meets annually at Stonehenge on Midsummer's Day.