interglacial and interstadial periods, more effective coupling occurred between the surface drainage network and the internal karst drainage system. Since limestone dissolves as the water percolates through its pores, an extended underground drainage system has developed, with caves and channels that enlarge with time when their roofs collapse, producing rocky exposures and perpendicular slopes, which is also the reason why the water is scarce. Only when an impenetrable stratum is met, does water appear on the surface. Aoös Gorge In the northern part of the Park, and very close to the town of Konitsa, the Aoös river passes through channels formed by the bulges of the nearby mountains of Trapezitsa , Mt Tymphe and Raidovouni , creating the Aoös Gorge that is long. It has an east-west direction and features numerous stone single-arched bridges from the 17th to 19th centuries, as well as monasteries built in the local architectural style. It is characterized by the great number of secondary gullies and currents, while the southern part of the gorge is steeper than the northern part. Deep and steep ravines within the perpendicular walls carry down into the Aoös large quantities of material resulting from limestone-weathering. The compact dolomites that lie on the bottom of the gorge date to the Early Jurassic period and are the oldest rock formations in the Park. Their age has been determined by means of sea fossils found inside them. Mt Tymphe Between the two gorges lies the Tymphe mountain range. Its highest peaks are Gamila , Astraka , Ploskos , and Lapatos . A unique feature of this area is natural pools formed by the erosion of local stream beds. A number of alpine lakes are found in the high-altitude zone of Mt Tymphe, surrounded by alpine pasture, which are home to several rare amphibian species. The biggest lake of the National Park, Drakolimni ("Dragonlake"), a formation that was created after the retreat of the glaciers, is located at a height of on Mt Tymphe. Its maximum depth is , while its surface covers . A number of vertical caves and precipices are found in the area around the village of Papingo, which lies near the Gamila and Astraka peaks. Some of them bear names inspired from mythology, such as the Hole of Odysseus and Chasm of Epos. These are being studied and explored by caving enthusiasts. The cave of "Provatina" ("Ewe's Cave"), with a