Fissure caves are formed by mechanic widening of thin fissures in the rocks, forming narrow, high, long, and mostly triangular shaped crevices. Often there are several, not connected, parallel crevices. Sometimes they open downwards, sometimes upwards. But if they are open upwards, with no roof, they are not really caves, they are more like gorges.
Most fissure caves are formed by forces related to erosion, along escarpments and cliff faces. Those caves are located above the ground water. If they are formed below the ground water level, they have a different character. In the deep, fissures often open very slowly, by slow movements of the crust caused generally by plate tectonics. As they are filled with ground water, the water transports soluted minerals, e.g. calcite or quartz, which are deposited very slow inside the fissure, forming minerals. If the amount of mineral growth is lower than the growth of the fissure, some space remains, which is continually filled with crystals. Small bubbles and holes are called geodes. Bigger holes, if they are big enough for people to crawl in, may be called caves. There is a big variety in size, form and orientation.