Pál-völgy Cave

Pál-völgy Cave is one of Budapest's tourist caves. The length developed for tourists is 500 m. It was an independent cave, but currently it is part of the Pál-völgy cave system. Pál-völgy Cave is located in the Buda Hills of the Danube–Ipoly National Park, in Budapest District II. The cave's surface protected area, about 4.7 ha, is under the management of the Danube–Ipoly National Park as a separately legislated, nationally significant nature protection area. It was one of Hungary's specially protected caves.

Description

The cave entrance is in Budapest District II, in a no‑longer‑worked, reclaimed, landscaped quarry next to Szépvölgyi út, under Szépvölgyi út number 162. There is a parking lot beside the quarry. Buses 65 and 65A departing from the Kolosy area stop at the quarry.

The cave length is 13 465 m. The cavity system is 1 020 m long in the K–Ny direction and 400 m wide in the É–D direction. Its vertical extent is 113.9 m, within which the deepest point is 86.7 m below the entrance, located in the Mosaic Hall of the Southern section, and the highest point is 27.2 m above the entrance, located in the Observation Hall on the side branch of the Dolphin corridor.

The extensive passage system includes a covering rock, similar to the other large caves of the area, mainly upper Eocene, shallow marine origin limestone (Szépvölgyi Limestone Formation), in which large foraminifera (Nummulites, Discocyclina) characteristic limestone frameworks are often present in rock-forming quantities. Alongside them, in some places the well-preserved fossils of shells (mainly Pecten species), sea urchins and bryozoans can also be observed on the cave walls, and even in the Lily Garden and the Heliktites Hall area marine lily anemone fragments are visible. The spatial location of the cave more or less follows the 25–30° dip of the Eocene limestone D–DK, therefore only a short upper passage level (Alba Regia corridor) and a few high, broken collapses (e.g., Ring Hall) penetrate into the limestone caprock, the upper Eocene marl. Of the limestone basement rocks, only the Eocene layer‑introducing, tuff‑debris base breccia is exposed in a few passages that extend deeper than the surrounding (e.g., Basement, Small Nut Branch).

The networked, in some places distinctly labyrinthine cavity system structure faithfully reflects the course of the fractures in the host rock, among which in the central zone comprising the December section, the Old Part and the Fourth Quarter, the ENE–DDN and EN–DK oriented, and in the eastern zone and the Southern section the EK–DN oriented, almost vertical fractures dominate. In the western zone represented by the Jubilee section area, the NE‑NE‑KDK oriented fractures and the flat displacement (sliding?) planes that determine the ceiling of certain passage sections (e.g., Explorer Branch, Small Dome, Emental Corridor) play a significant role.

In addition to the tectonic preformation, the alteration of the host rock due to post‑volcanic, hydrothermal processes may have played a role in the designation of the passage locations, which transformed the limestone along individual fault lines into porous, loose, high‑sulphur‑content material in zones ranging from a few decimetres to 1–1.5 m in width. These sulphur zones occur at the main level of most passages and are very well visible at the higher levels above approximately 160 m above sea level (e.g. Upper‑step‑gallery, Dolphin‑gallery, Great‑wall), where their white material is sharply bounded by a rust‑brown edge against the yellowish colour of the host rock.

The cave’s spatial structure and dissolution forms both show cavity dissolution in a phreatic zone below the karst water table, whose principal driving factor was probably mixing corrosion occurring in the zone where warm to hot waters rising from depth along fault lines met descending cold karst waters coming from the open karst surfaces of the mountain range. The beginning of the formation of the passage system representing the ancient watercourses of the ancestors of the present‑day Danube‑side springs may have been at the end of the lower Pleistocene (about 1 million years ago). With the uplift of the Buda Hills and the deeper incision of the Danube, the karst water table gradually sank, and most of the passages that became dry consequently were already air‑filled at least 350 000 years ago, based on the dating of the water‑level indicator deposits located in the zone around 160 m above sea level.

The cave is currently inactive from a hydrological point of view. Its deepest point (Mosaic Hall) is also located above the karst water level, but when this local, hanging karst water-type storage level rises (in which, according to observations, the ruptures of water pipes from the built human environment probably also cooperate) a lake up to 3 m deep can even form here. Although the cave (or at least parts of it) in the past was certainly also affected by surface inflow waters associated with the incising of Szép Valley. These mainly played a role in filling the passages and in the accumulation of their sediments, but did not perform any substantial cavity‑enlarging work. Dripping waters, as well as the shallow pools that collect from them, are still observable in many places today, and they also contribute to the cave’s infill by their stalactite formation.

The tectonic preparation can be traced not only in the cave’s plan but also in the essentially fissure‑type development of most corridors. The passages are relatively narrow, on average 2 m wide, but their height is significant, reaching 15–20 m in some places. The passage cross‑sections, however, are mostly characteristically asymmetrical, shaped like the letters b and d. Narrow chimneys formed by the upward thrust of ceiling zones connect to the lower dissolved cross‑section parts that are eroded opposite to the direction of rock dissolution. This phenomenon is most clearly observed in the Fourth Quarter, the highest and oldest part of the cave (e.g., Nyest Corridor, Dolphin Corridor), but it is also detectable in a less developed form in the passages of the deepest zone.