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(Geomorphology)

Slovenian for "field", a polje resembles a long, narrow valley, but is an especially large sinkhole -- one large enough for people to settle in. They seem so magical to me that I half-expect to find elves living in them. Another name is "karst window".

As might be expected from the name, numerous poljes can be found in the Kraš region of Slovenia where karst processes were first studied. But they can be found in many areas of karst development throughout the world, where they are an intermediate stage in the erosion of layers of massive limestone: Croatia, Bosnia, Belize, Missouri, Kosovo, Vietnam, Kentucky, South China, Tennessee.

A polje is much longer than it is wide, has a flat bottom, and always has a stream running its length (A karst window whose stream has dried up is a "dry valley"). Some poljes are "open", meaning that their drainage is on the surface, but the more interesting "closed poljes" have underground drainage. The stream comes out of a cave or karst spring at one end, and disappears into a cave or a special type of sinkhole (a ponor) at the other. Occasionally, a residual hill of limestone, or hum, will poke up through the valley floor. The bottom is quite fertile, and large enough poljes (some in Slovenia are 50 km long and 6 km wide) support agriculture and settlement.

Poljes appear at an intermediate stage of karst development, after dolines, caves, and sinkholes, and before the final stages of tower karst and the complete erosion of the limestone. The mechanisms of polje formation are only partly understood. One way to think of it, however, is that there is an underground drainage system in the limestone, and that drainage system is revealed little by little as the caves it runs through collapse.

As in all karst development, caves form in areas of massive limestone permeated by groundwater. Sometimes, a structural feature, such as a fault or a syncline, causes a long chain of caves to form. If the water table drops to a point just above the cave system's floor, a stream forms down its length. Eventually part of the cave system collapses, revealing the polje.

As the landscape develops further, chains of poljes can coalesce into larger poljes. At the final stage of the connection of two, you will sometimes find a natural bridge with the common stream running beneath.

Remember, we're dealing with geomorphology jargon here. Shallot points out that in Slovenian, Croatian, and other South Slavic languages, "polje" means simply "field" -- to name one of these things in Slovenian, you'd say "kraško polje", "karst field".


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